diff --git a/tests/unit/test_auth_service.py b/tests/unit/test_auth_service.py
index 42d1c64bc..5e5464a3c 100755
--- a/tests/unit/test_auth_service.py
+++ b/tests/unit/test_auth_service.py
@@ -458,8 +458,15 @@ class TestJWTService:
     def test_verify_token_tampered_signature(self, service):
         """篡改签名的token无法验证"""
         token = service.create_access_token(user_id="user_123")
-        # 篡改最后一个字符
-        tampered = token[:-1] + ("A" if token[-1] != "A" else "B")
+        # 篡改签名部分中间的字符（JWT格式: header.payload.signature）
+        # 改最后一个字符不可靠（可能是padding），改签名中间确保验证失败
+        parts = token.rsplit(".", 1)
+        assert len(parts) == 2, "JWT格式不对，应该有两个点分隔三部分"
+        signature = parts[1]
+        # 篡改签名中间的几个字符
+        mid = len(signature) // 2
+        tampered_sig = signature[:mid] + ("X" if signature[mid] != "X" else "Y") + signature[mid + 1 :]
+        tampered = parts[0] + "." + tampered_sig
         with pytest.raises(InvalidTokenError):
             service.verify_token(tampered)