diff --git a/src/antlr4/SysY.g4 b/src/antlr4/SysY.g4
index c22ab5f..82a624f 100644
--- a/src/antlr4/SysY.g4
+++ b/src/antlr4/SysY.g4
@@ -1,35 +1,73 @@
+// SysY 子集语法：支持形如
+// int main() { int a = 1; int b = 2; return a + b; }
+// 的最小返回表达式编译。
+
+// 后续需要自行添加
 grammar SysY;
 
-// 说明：
-// - 这是一个“最小可用”的 SysY.g4，用于避免空文件导致的 ANTLR 解析报错。
-// - 后续请按 SysY 语言规范逐步补全 lexer/parser 规则。
-// - 本工程约定：ANTLR 生成的 C++ 源码/头文件不进入仓库，统一生成到构建目录（例如 build/generated/antlr4/）。
+compUnit
+    : funcDef EOF
+    ;
+
+funcDef
+    : Int Main L_PAREN R_PAREN block
+    ;
+
+block
+    : L_BRACE stmt* R_BRACE
+    ;
+
+stmt
+    : varDecl
+    | returnStmt
+    ;
+
+varDecl
+    : Int Ident (Assign exp)? Semi
+    ;
+
+returnStmt
+    : Return exp Semi
+    ;
+
+exp
+    : addExp
+    ;
 
-compilationUnit
-  : (statement)* EOF
-  ;
+addExp
+    : primary (AddOp primary)*
+    ;
 
-statement
-  : 'return' expression? ';'
-  | ';'
-  ;
+primary
+    : Number
+    | Ident
+    | L_PAREN exp R_PAREN
+    ;
 
-expression
-  : IntegerLiteral
-  | Identifier
-  ;
+Int     : 'int';
+Return  : 'return';
+Main    : 'main';
 
-// -------- lexer --------
+AddOp   : '+';
+Assign  : '=';
+Semi    : ';';
+L_PAREN : '(';
+R_PAREN : ')';
+L_BRACE : '{';
+R_BRACE : '}';
 
-IntegerLiteral
-  : [0-9]+
-  ;
+Ident
+    : [a-zA-Z_][a-zA-Z_0-9]*
+    ;
 
-Identifier
-  : [a-zA-Z_] [a-zA-Z0-9_]*
-  ;
+Number
+    : [0-9]+
+    ;
 
-Whitespace
-  : [ \t\r\n]+ -> skip
-  ;
+WS
+    : [ \t\r\n]+ -> skip
+    ;
 
+COMMENT
+    : '//' ~[\r\n]* -> skip
+    ;
diff --git a/src/ast/AstNodes.cpp b/src/ast/AstNodes.cpp
index 39c5b37..5493907 100644
--- a/src/ast/AstNodes.cpp
+++ b/src/ast/AstNodes.cpp
@@ -1,4 +1,5 @@
+
 // AST 节点定义与实现：
 // - 表达式、语句、声明、函数、类型等节点
 // - 支持后续阶段在节点上附加信息（类型、符号绑定、常量值等）
-
+#include "ast/AstNodes.h"
diff --git a/src/ast/AstNodes.h b/src/ast/AstNodes.h
new file mode 100644
index 0000000..5433cd1
--- /dev/null
+++ b/src/ast/AstNodes.h
@@ -0,0 +1,70 @@
+
+#pragma once
+
+#include <memory>
+#include <string>
+#include <vector>
+
+namespace ast {
+
+enum class BinaryOp { Add, Sub, Mul, Div };
+
+struct Expr {
+  virtual ~Expr() = default;
+};
+
+struct NumberExpr : Expr {
+  int value{};
+  explicit NumberExpr(int v) : value(v) {}
+};
+
+struct VarExpr : Expr {
+  std::string name;
+  explicit VarExpr(std::string n) : name(std::move(n)) {}
+};
+
+struct BinaryExpr : Expr {
+  BinaryOp op;
+  std::shared_ptr<Expr> lhs;
+  std::shared_ptr<Expr> rhs;
+  BinaryExpr(BinaryOp op, std::shared_ptr<Expr> lhs, std::shared_ptr<Expr> rhs)
+      : op(op), lhs(std::move(lhs)), rhs(std::move(rhs)) {}
+};
+
+struct Stmt {
+  virtual ~Stmt() = default;
+};
+
+struct ReturnStmt : Stmt {
+  std::shared_ptr<Expr> value;
+  explicit ReturnStmt(std::shared_ptr<Expr> v) : value(std::move(v)) {}
+};
+
+struct VarDecl {
+  std::string name;
+  std::shared_ptr<Expr> init;  // nullptr if no initializer
+  VarDecl(std::string n, std::shared_ptr<Expr> i)
+      : name(std::move(n)), init(std::move(i)) {}
+};
+
+struct Block {
+  std::vector<std::shared_ptr<VarDecl>> varDecls;
+  std::vector<std::shared_ptr<Stmt>> stmts;
+};
+
+struct FuncDef {
+  std::string name;
+  std::shared_ptr<Block> body;
+  FuncDef(std::string n, std::shared_ptr<Block> b)
+      : name(std::move(n)), body(std::move(b)) {}
+};
+
+struct CompUnit {
+  std::shared_ptr<FuncDef> func;
+  explicit CompUnit(std::shared_ptr<FuncDef> f) : func(std::move(f)) {}
+};
+
+// 调试打印
+void PrintAST(const CompUnit& cu);
+
+}  // namespace ast
diff --git a/src/ast/AstPrinter.cpp b/src/ast/AstPrinter.cpp
index 1e37684..d3e71b1 100644
--- a/src/ast/AstPrinter.cpp
+++ b/src/ast/AstPrinter.cpp
@@ -1,4 +1,72 @@
-// AST 调试打印：
-// - 以可读形式打印 AST 结构
-// - 用于验证 AST 构建与语义分析结果，便于定位问题
+// 简单 AST 调试打印，便于前端验证。
 
+#include "ast/AstNodes.h"
+
+#include <iostream>
+
+namespace ast {
+
+static void PrintExpr(const Expr* expr);
+
+static void PrintIndent(int depth) {
+  for (int i = 0; i < depth; ++i) std::cout << "  ";
+}
+
+static void PrintExpr(const Expr* expr) {
+  if (auto num = dynamic_cast<const NumberExpr*>(expr)) {
+    std::cout << num->value;
+  } else if (auto var = dynamic_cast<const VarExpr*>(expr)) {
+    std::cout << var->name;
+  } else if (auto bin = dynamic_cast<const BinaryExpr*>(expr)) {
+    std::cout << "(";
+    PrintExpr(bin->lhs.get());
+    const char* op = "?";
+    switch (bin->op) {
+      case BinaryOp::Add:
+        op = "+";
+        break;
+      case BinaryOp::Sub:
+        op = "-";
+        break;
+      case BinaryOp::Mul:
+        op = "*";
+        break;
+      case BinaryOp::Div:
+        op = "/";
+        break;
+    }
+    std::cout << " " << op << " ";
+    PrintExpr(bin->rhs.get());
+    std::cout << ")";
+  }
+}
+
+void PrintAST(const CompUnit& cu) {
+  if (!cu.func) return;
+  std::cout << "func " << cu.func->name << " () {\n";
+  const auto& body = cu.func->body;
+  if (!body) {
+    std::cout << "}\n";
+    return;
+  }
+  for (const auto& decl : body->varDecls) {
+    PrintIndent(1);
+    std::cout << "var " << decl->name;
+    if (decl->init) {
+      std::cout << " = ";
+      PrintExpr(decl->init.get());
+    }
+    std::cout << ";\n";
+  }
+  for (const auto& stmt : body->stmts) {
+    if (auto ret = dynamic_cast<ReturnStmt*>(stmt.get())) {
+      PrintIndent(1);
+      std::cout << "return ";
+      PrintExpr(ret->value.get());
+      std::cout << ";\n";
+    }
+  }
+  std::cout << "}\n";
+}
+
+}  // namespace ast
diff --git a/src/frontend/AntlrDriver.cpp b/src/frontend/AntlrDriver.cpp
index 571511a..29eb1c3 100644
--- a/src/frontend/AntlrDriver.cpp
+++ b/src/frontend/AntlrDriver.cpp
@@ -1,5 +1,34 @@
-// 前端解析驱动：
-// - 读取源代码
-// - 调用 ANTLR 生成的 lexer/parser 得到 parse tree
-// - 对外提供“可用的解析入口”（语法正确性由测试保证）
+// 调用 ANTLR 生成的 Lexer/Parser，返回 parse tree。
+#include "frontend/AntlrDriver.h"
 
+#include <fstream>
+#include <sstream>
+#include <stdexcept>
+
+#include "SysYLexer.h"
+#include "SysYParser.h"
+#include "antlr4-runtime.h"
+
+AntlrResult ParseFileWithAntlr(const std::string& path) {
+  std::ifstream fin(path);
+  if (!fin.is_open()) {
+    throw std::runtime_error("无法打开输入文件: " + path);
+  }
+  std::ostringstream ss;
+  ss << fin.rdbuf();
+
+  auto input = std::make_unique<antlr4::ANTLRInputStream>(ss.str());
+  auto lexer = std::make_unique<SysYLexer>(input.get());
+  auto tokens = std::make_unique<antlr4::CommonTokenStream>(lexer.get());
+  auto parser = std::make_unique<SysYParser>(tokens.get());
+  parser->removeErrorListeners();
+  auto tree = parser->compUnit();
+
+  AntlrResult result;
+  result.input = std::move(input);
+  result.lexer = std::move(lexer);
+  result.tokens = std::move(tokens);
+  result.parser = std::move(parser);
+  result.tree = tree;
+  return result;
+}
diff --git a/src/frontend/AntlrDriver.h b/src/frontend/AntlrDriver.h
new file mode 100644
index 0000000..ee22da9
--- /dev/null
+++ b/src/frontend/AntlrDriver.h
@@ -0,0 +1,20 @@
+// 包装 ANTLR4，提供简易的解析入口。
+#pragma once
+
+#include <memory>
+#include <string>
+
+#include "SysYLexer.h"
+#include "SysYParser.h"
+#include "antlr4-runtime.h"
+
+struct AntlrResult {
+  std::unique_ptr<antlr4::ANTLRInputStream> input;
+  std::unique_ptr<SysYLexer> lexer;
+  std::unique_ptr<antlr4::CommonTokenStream> tokens;
+  std::unique_ptr<SysYParser> parser;
+  antlr4::tree::ParseTree* tree = nullptr;  // owned by parser
+};
+
+// 解析指定文件，发生错误时抛出 std::runtime_error。
+AntlrResult ParseFileWithAntlr(const std::string& path);
diff --git a/src/frontend/AstBuilder.cpp b/src/frontend/AstBuilder.cpp
index e57235e..da6b3d9 100644
--- a/src/frontend/AstBuilder.cpp
+++ b/src/frontend/AstBuilder.cpp
@@ -1,4 +1,114 @@
-// AST 构建：
-// - 将 ANTLR parse tree 转换为 AST（对应 src/ast/*）
-// - 在 AST 节点上保留必要的定位信息（可选，用于调试/日志）
+// 将 parse tree 转换为 AST。
+#include "frontend/AstBuilder.h"
 
+#include <any>
+#include <stdexcept>
+#include <string>
+#include <utility>
+#include <vector>
+
+#include "SysYBaseVisitor.h"
+#include "SysYParser.h"
+#include "ast/AstNodes.h"
+#include "antlr4-runtime.h"
+
+namespace {
+
+using ast::BinaryExpr;
+using ast::BinaryOp;
+using ast::Block;
+using ast::CompUnit;
+using ast::FuncDef;
+using ast::NumberExpr;
+using ast::ReturnStmt;
+using ast::VarDecl;
+using ast::VarExpr;
+
+template <typename T>
+T Take(std::any&& value) {
+  if (auto* ptr = std::any_cast<T>(&value)) {
+    return std::move(*ptr);
+  }
+  throw std::runtime_error("AST 构建失败：类型不匹配");
+}
+
+class Builder : public SysYBaseVisitor {
+ public:
+  std::any visitCompUnit(SysYParser::CompUnitContext* ctx) override {
+    auto func = Take<std::shared_ptr<FuncDef>>(visit(ctx->funcDef()));
+    return std::make_shared<CompUnit>(std::move(func));
+  }
+
+  std::any visitFuncDef(SysYParser::FuncDefContext* ctx) override {
+    auto body = Take<std::shared_ptr<Block>>(visit(ctx->block()));
+    return std::make_shared<FuncDef>("main", std::move(body));
+  }
+
+  std::any visitBlock(SysYParser::BlockContext* ctx) override {
+    auto block = std::make_shared<Block>();
+    for (auto stmtCtx : ctx->stmt()) {
+      if (stmtCtx->varDecl()) {
+        block->varDecls.emplace_back(
+            Take<std::shared_ptr<VarDecl>>(visit(stmtCtx->varDecl())));
+      } else if (stmtCtx->returnStmt()) {
+        block->stmts.emplace_back(
+            Take<std::shared_ptr<ReturnStmt>>(visit(stmtCtx->returnStmt())));
+      }
+    }
+    return block;
+  }
+
+  std::any visitVarDecl(SysYParser::VarDeclContext* ctx) override {
+    std::shared_ptr<ast::Expr> init;
+    if (ctx->exp()) {
+      init = Take<std::shared_ptr<ast::Expr>>(visit(ctx->exp()));
+    }
+    return std::make_shared<VarDecl>(ctx->Ident()->getText(), std::move(init));
+  }
+
+  std::any visitReturnStmt(SysYParser::ReturnStmtContext* ctx) override {
+    auto expr = Take<std::shared_ptr<ast::Expr>>(visit(ctx->exp()));
+    return std::make_shared<ReturnStmt>(std::move(expr));
+  }
+
+  std::any visitExp(SysYParser::ExpContext* ctx) override {
+    return visit(ctx->addExp());
+  }
+
+  std::any visitAddExp(SysYParser::AddExpContext* ctx) override {
+    auto node = Take<std::shared_ptr<ast::Expr>>(visit(ctx->primary(0)));
+    for (size_t i = 1; i < ctx->primary().size(); ++i) {
+      auto rhs = Take<std::shared_ptr<ast::Expr>>(visit(ctx->primary(i)));
+      auto opToken = ctx->AddOp(i - 1);
+      BinaryOp op = BinaryOp::Add;
+      if (opToken->getText() == "-") op = BinaryOp::Sub;
+      node = std::make_shared<BinaryExpr>(op, std::move(node), std::move(rhs));
+    }
+    return node;
+  }
+
+  std::any visitPrimary(SysYParser::PrimaryContext* ctx) override {
+    if (ctx->Number()) {
+      std::shared_ptr<ast::Expr> expr =
+          std::make_shared<NumberExpr>(std::stoi(ctx->Number()->getText()));
+      return expr;
+    }
+    if (ctx->Ident()) {
+      std::shared_ptr<ast::Expr> expr =
+          std::make_shared<VarExpr>(ctx->Ident()->getText());
+      return expr;
+    }
+    return visit(ctx->exp());
+  }
+};
+
+}  // namespace
+
+std::shared_ptr<ast::CompUnit> BuildAst(antlr4::tree::ParseTree* tree) {
+  if (!tree) {
+    throw std::runtime_error("parse tree 为空");
+  }
+  Builder visitor;
+  auto result = visitor.visit(tree);
+  return Take<std::shared_ptr<ast::CompUnit>>(std::move(result));
+}
diff --git a/src/frontend/AstBuilder.h b/src/frontend/AstBuilder.h
new file mode 100644
index 0000000..7607667
--- /dev/null
+++ b/src/frontend/AstBuilder.h
@@ -0,0 +1,16 @@
+// 将 ANTLR parse tree 转换为内部 AST。
+#pragma once
+
+#include <memory>
+
+namespace antlr4 {
+namespace tree {
+class ParseTree;
+}
+}  // namespace antlr4
+
+namespace ast {
+struct CompUnit;
+}
+
+std::shared_ptr<ast::CompUnit> BuildAst(antlr4::tree::ParseTree* tree);