diff --git a/lib/calculator.dart b/lib/calculator.dart
index c2792b2..e29e5f9 100644
--- a/lib/calculator.dart
+++ b/lib/calculator.dart
@@ -7,6 +7,9 @@ abstract class Expr {
   /// 新增：对表达式进行“求值/数值化”——尽可能把可算的部分算出来
   Expr evaluate();
 
+  /// Substitute variable with value
+  Expr substitute(String varName, Expr value);
+
   @override
   String toString();
 
@@ -27,6 +30,9 @@ class IntExpr extends Expr {
   @override
   Expr evaluate() => this;
 
+  @override
+  Expr substitute(String varName, Expr value) => this;
+
   @override
   String toString() => value.toString();
 }
@@ -42,10 +48,31 @@ class DoubleExpr extends Expr {
   @override
   Expr evaluate() => this;
 
+  @override
+  Expr substitute(String varName, Expr value) => this;
+
   @override
   String toString() => value.toString();
 }
 
+// === VarExpr ===
+class VarExpr extends Expr {
+  final String name;
+  VarExpr(this.name);
+
+  @override
+  Expr simplify() => this;
+
+  @override
+  Expr evaluate() => this;
+
+  @override
+  Expr substitute(String varName, Expr value) => name == varName ? value : this;
+
+  @override
+  String toString() => name;
+}
+
 // === FractionExpr.evaluate ===
 class FractionExpr extends Expr {
   final int numerator;
@@ -74,6 +101,9 @@ class FractionExpr extends Expr {
   @override
   Expr evaluate() => simplify();
 
+  @override
+  Expr substitute(String varName, Expr value) => this;
+
   @override
   String toString() => "$numerator/$denominator";
 }
@@ -150,6 +180,12 @@ class AddExpr extends Expr {
     return AddExpr(l, r);
   }
 
+  @override
+  Expr substitute(String varName, Expr value) => AddExpr(
+    left.substitute(varName, value),
+    right.substitute(varName, value),
+  );
+
   @override
   String toString() => "($left + $right)";
 }
@@ -213,6 +249,12 @@ class SubExpr extends Expr {
     return SubExpr(l, r);
   }
 
+  @override
+  Expr substitute(String varName, Expr value) => SubExpr(
+    left.substitute(varName, value),
+    right.substitute(varName, value),
+  );
+
   @override
   String toString() => "($left - $right)";
 }
@@ -296,6 +338,12 @@ class MulExpr extends Expr {
     return MulExpr(l, r);
   }
 
+  @override
+  Expr substitute(String varName, Expr value) => MulExpr(
+    left.substitute(varName, value),
+    right.substitute(varName, value),
+  );
+
   @override
   String toString() => "($left * $right)";
 }
@@ -378,6 +426,12 @@ class DivExpr extends Expr {
     return DivExpr(l, r);
   }
 
+  @override
+  Expr substitute(String varName, Expr value) => DivExpr(
+    left.substitute(varName, value),
+    right.substitute(varName, value),
+  );
+
   @override
   String toString() => "($left / $right)";
 }
@@ -429,6 +483,10 @@ class SqrtExpr extends Expr {
     return SqrtExpr(i);
   }
 
+  @override
+  Expr substitute(String varName, Expr value) =>
+      SqrtExpr(inner.substitute(varName, value));
+
   @override
   String toString() => "sqrt($inner)";
 }
@@ -456,6 +514,10 @@ class CosExpr extends Expr {
     return CosExpr(i);
   }
 
+  @override
+  Expr substitute(String varName, Expr value) =>
+      CosExpr(inner.substitute(varName, value));
+
   @override
   String toString() => "cos($inner)";
 }
@@ -483,6 +545,10 @@ class SinExpr extends Expr {
     return SinExpr(i);
   }
 
+  @override
+  Expr substitute(String varName, Expr value) =>
+      SinExpr(inner.substitute(varName, value));
+
   @override
   String toString() => "sin($inner)";
 }
@@ -510,6 +576,10 @@ class TanExpr extends Expr {
     return TanExpr(i);
   }
 
+  @override
+  Expr substitute(String varName, Expr value) =>
+      TanExpr(inner.substitute(varName, value));
+
   @override
   String toString() => "tan($inner)";
 }
diff --git a/lib/parser.dart b/lib/parser.dart
index f280afc..39555ae 100644
--- a/lib/parser.dart
+++ b/lib/parser.dart
@@ -99,6 +99,13 @@ class Parser {
       return TanExpr(inner);
     }
 
+    // 解析变量 (单个字母)
+    if (RegExp(r'[a-zA-Z]').hasMatch(current)) {
+      var varName = current;
+      eat();
+      return VarExpr(varName);
+    }
+
     // 解析整数
     var buf = '';
     while (!isEnd && RegExp(r'\d').hasMatch(current)) {
diff --git a/lib/screens/calculator_home_page.dart b/lib/screens/calculator_home_page.dart
index 8c744aa..d3cd51e 100644
--- a/lib/screens/calculator_home_page.dart
+++ b/lib/screens/calculator_home_page.dart
@@ -3,7 +3,8 @@ import 'package:latext/latext.dart';
 import 'package:simple_math_calc/models/calculation_step.dart';
 import 'package:simple_math_calc/solver.dart';
 import 'package:fl_chart/fl_chart.dart';
-import 'package:math_expressions/math_expressions.dart' as math_expressions;
+import 'package:simple_math_calc/calculator.dart';
+import 'package:simple_math_calc/parser.dart';
 import 'dart:math';
 
 class CalculatorHomePage extends StatefulWidget {
@@ -70,11 +71,8 @@ class _CalculatorHomePageState extends State<CalculatorHomePage> {
       );
 
       // 解析表达式
-      final parser = math_expressions.ShuntingYardParser();
-      final expr = parser.parse(functionExpr);
-
-      // 创建变量 x
-      final x = math_expressions.Variable('x');
+      final parser = Parser(functionExpr);
+      final expr = parser.parse();
 
       // 根据缩放因子动态调整范围和步长
       final range = 10.0 * zoomFactor;
@@ -84,13 +82,15 @@ class _CalculatorHomePageState extends State<CalculatorHomePage> {
       List<FlSpot> points = [];
       for (double i = -range; i <= range; i += step) {
         try {
-          final context = math_expressions.ContextModel()
-            ..bindVariable(x, math_expressions.Number(i));
-          final evaluator = math_expressions.RealEvaluator(context);
-          final y = evaluator.evaluate(expr);
+          // 替换变量 x 为当前值
+          final substituted = expr.substitute('x', DoubleExpr(i));
+          final evaluated = substituted.evaluate();
 
-          if (y.isFinite && !y.isNaN) {
-            points.add(FlSpot(i, y.toDouble()));
+          if (evaluated is DoubleExpr) {
+            final y = evaluated.value;
+            if (y.isFinite && !y.isNaN) {
+              points.add(FlSpot(i, y));
+            }
           }
         } catch (e) {
           // 跳过无法计算的点
diff --git a/web/index.html b/web/index.html
index 9b07bee..33b95b3 100644
--- a/web/index.html
+++ b/web/index.html
@@ -16,7 +16,7 @@
 
   <meta charset="UTF-8">
   <meta content="IE=Edge" http-equiv="X-UA-Compatible">
-  <meta name="description" content="A new Flutter project.">
+  <meta name="description" content="A simple math calculator.">
 
   <!-- iOS meta tags & icons -->
   <meta name="mobile-web-app-capable" content="yes">