diff --git a/ExpenseAPI/Controllers/ExpenseController.cs b/ExpenseAPI/Controllers/ExpenseController.cs
index f165592..27c42d3 100644
--- a/ExpenseAPI/Controllers/ExpenseController.cs
+++ b/ExpenseAPI/Controllers/ExpenseController.cs
@@ -78,32 +78,53 @@ public async Task<ActionResult<Expense>> PostExpense(Expense expense)
         [HttpPut("{id}")]
         public async Task<IActionResult> PutExpense(int id, Expense expense)
         {
-            if (id != expense.Id)
+            if (expense.Id != 0 && id != expense.Id)
             {
                 _logger.LogWarning("Mismatched expense id for update. Route id: {RouteId}, Expense id: {ExpenseId}", id, expense.Id);
                 return BadRequest();
             }
 
-            _context.Entry(expense).State = EntityState.Modified;
+            var existingExpense = await _context.Expenses.FindAsync(id);
+            if (existingExpense == null)
+            {
+                _logger.LogWarning("Expense with id: {ExpenseId} not found during update", id);
+                return NotFound();
+            }
+
+            if (expense.Date != default)
+            {
+                existingExpense.Date = expense.Date;
+            }
+
+            if (expense.Description is not null)
+            {
+                existingExpense.Description = expense.Description;
+            }
+
+            existingExpense.Amount = expense.Amount;
+            existingExpense.Category = expense.Category;
+            existingExpense.Title = expense.Title;
 
-            try
+            if (string.IsNullOrWhiteSpace(existingExpense.Description))
             {
-                await _context.SaveChangesAsync();
-                _logger.LogInformation("Updated expense with id: {ExpenseId}", id);
+                return BadRequest("Description is required");
             }
-            catch (DbUpdateConcurrencyException)
+
+            if (existingExpense.Description == "午餐" && existingExpense.Amount > 400)
+            {
+                _logger.LogWarning("Lunch expense with amount over 400 is not allowed");
+                return BadRequest("午餐費用不能超過400元");
+            }
+
+            if (existingExpense.Amount < 0)
             {
-                if (!ExpenseExists(id))
-                {
-                    _logger.LogWarning("Expense with id: {ExpenseId} not found during update", id);
-                    return NotFound();
-                }
-                else
-                {
-                    throw;
-                }
+                _logger.LogWarning("數量不能為負的");
+                return BadRequest("數量不能為負的");
             }
 
+            await _context.SaveChangesAsync();
+            _logger.LogInformation("Updated expense with id: {ExpenseId}", id);
+
             return NoContent();
         }
 
@@ -130,4 +151,4 @@ private bool ExpenseExists(int id)
             return _context.Expenses.Any(e => e.Id == id);
         }
     }
-}
\ No newline at end of file
+}
diff --git a/ExpenseApiTest/ExpenseApiTest.cs b/ExpenseApiTest/ExpenseApiTest.cs
index 49ffebf..f61f6d0 100644
--- a/ExpenseApiTest/ExpenseApiTest.cs
+++ b/ExpenseApiTest/ExpenseApiTest.cs
@@ -100,4 +100,58 @@ public async Task PostExpense_BoundaryExpense_ReturnsCreatedExpense()
         Assert.Equal(newExpense.Date, createdExpense.Date);
         Assert.Equal(newExpense.Title, createdExpense.Title);
     }
+
+    [Fact]
+    public async Task PutExpense_PartialUpdateWithoutDate_KeepsOriginalDate()
+    {
+        // Arrange
+        var existingExpense = new Expense
+        {
+            Category = "食",
+            Description = "早餐",
+            Amount = 80,
+            Date = DateTime.Parse("2026-01-15"),
+            Title = "早餐"
+        };
+
+        _context.Expenses.Add(existingExpense);
+        await _context.SaveChangesAsync();
+
+        var updateRequest = new Expense
+        {
+            Description = "午餐",
+            Amount = 120
+        };
+
+        // Act
+        var result = await _controller.PutExpense(existingExpense.Id, updateRequest);
+        var updatedExpense = await _context.Expenses.FindAsync(existingExpense.Id);
+
+        // Assert
+        Assert.IsType<NoContentResult>(result);
+        Assert.NotNull(updatedExpense);
+        Assert.Equal(DateTime.Parse("2026-01-15"), updatedExpense!.Date);
+        Assert.Equal("午餐", updatedExpense.Description);
+        Assert.Equal(120, updatedExpense.Amount);
+    }
+
+    [Fact]
+    public async Task PutExpense_MismatchedId_ReturnsBadRequest()
+    {
+        // Arrange
+        var updateRequest = new Expense
+        {
+            Id = 99,
+            Description = "晚餐",
+            Amount = 100,
+            Date = DateTime.Parse("2026-01-10")
+        };
+
+        // Act
+        var result = await _controller.PutExpense(1, updateRequest);
+
+        // Assert
+        Assert.IsType<BadRequestResult>(result);
+    }
+
 }
\ No newline at end of file