diff --git a/nonebot/compat.py b/nonebot/compat.py
index a2c96968..582ec07f 100644
--- a/nonebot/compat.py
+++ b/nonebot/compat.py
@@ -11,7 +11,7 @@ FrontMatter:
 
 from collections.abc import Generator
 from dataclasses import dataclass, is_dataclass
-from functools import cached_property
+from functools import cached_property, wraps
 from typing import (
     TYPE_CHECKING,
     Annotated,
@@ -25,13 +25,14 @@ from typing import (
     Union,
     overload,
 )
-from typing_extensions import Self, get_args, get_origin, is_typeddict
+from typing_extensions import ParamSpec, Self, get_args, get_origin, is_typeddict
 
 from pydantic import VERSION, BaseModel
 
 from nonebot.typing import origin_is_annotated
 
 T = TypeVar("T")
+P = ParamSpec("P")
 
 PYDANTIC_V2 = int(VERSION.split(".", 1)[0]) == 2
 
@@ -49,6 +50,7 @@ __all__ = (
     "PYDANTIC_V2",
     "ConfigDict",
     "FieldInfo",
+    "LegacyUnionField",
     "ModelField",
     "PydanticUndefined",
     "PydanticUndefinedType",
@@ -71,7 +73,7 @@ __autodoc__ = {
 
 
 if PYDANTIC_V2:  # pragma: pydantic-v2
-    from pydantic import GetCoreSchemaHandler
+    from pydantic import Field, GetCoreSchemaHandler
     from pydantic import TypeAdapter as TypeAdapter
     from pydantic import field_validator as field_validator
     from pydantic import model_validator as model_validator
@@ -94,6 +96,17 @@ if PYDANTIC_V2:  # pragma: pydantic-v2
     DEFAULT_CONFIG = ConfigDict(extra="allow", arbitrary_types_allowed=True)
     """Default config for validations"""
 
+    def _get_legacy_union_field(func: Callable[P, T]) -> Callable[P, T]:
+        @wraps(func)
+        def wrapper(*args: P.args, **kwargs: P.kwargs) -> T:
+            kwargs["union_mode"] = "left_to_right"
+            return func(*args, **kwargs)
+
+        return wrapper
+
+    LegacyUnionField = _get_legacy_union_field(Field)
+    LegacyUnionField.__doc__ = "Mark field to use legacy left to right union mode"
+
     class FieldInfo(BaseFieldInfo):  # pyright: ignore[reportGeneralTypeIssues]
         """FieldInfo class with extra property for compatibility with pydantic v1"""
 
@@ -292,6 +305,8 @@ else:  # pragma: pydantic-v1
         extra = Extra.allow
         arbitrary_types_allowed = True
 
+    from pydantic.fields import Field as LegacyUnionField
+
     class FieldInfo(BaseFieldInfo):
         def __init__(self, default: Any = PydanticUndefined, **kwargs: Any):
             # preprocess default value to make it compatible with pydantic v2
diff --git a/nonebot/config.py b/nonebot/config.py
index dc2dba94..8ea08ded 100644
--- a/nonebot/config.py
+++ b/nonebot/config.py
@@ -30,6 +30,7 @@ from pydantic.networks import IPvAnyAddress
 from nonebot.compat import (
     PYDANTIC_V2,
     ConfigDict,
+    LegacyUnionField,
     ModelField,
     PydanticUndefined,
     PydanticUndefinedType,
@@ -424,7 +425,7 @@ class Config(BaseSettings):
     """NoneBot {ref}`nonebot.drivers.ReverseDriver` 服务端监听的 IP/主机名。"""
     port: int = Field(default=8080, ge=1, le=65535)
     """NoneBot {ref}`nonebot.drivers.ReverseDriver` 服务端监听的端口。"""
-    log_level: Union[int, str] = "INFO"
+    log_level: Union[int, str] = LegacyUnionField(default="INFO")
     """NoneBot 日志输出等级，可以为 `int` 类型等级或等级名称。
 
     参考 [记录日志](https://nonebot.dev/docs/appendices/log)，[loguru 日志等级](https://loguru.readthedocs.io/en/stable/api/logger.html#levels)。
diff --git a/tests/.env.example b/tests/.env.example
index d6d5b109..dc4f855b 100644
--- a/tests/.env.example
+++ b/tests/.env.example
@@ -1,4 +1,5 @@
 SIMPLE=simple
+int_str=123
 COMPLEX='
 [1, 2, 3]
 '
diff --git a/tests/test_config.py b/tests/test_config.py
index 3ad9bbf4..345426af 100644
--- a/tests/test_config.py
+++ b/tests/test_config.py
@@ -3,7 +3,7 @@ from typing import TYPE_CHECKING, Optional, Union
 from pydantic import BaseModel, Field
 import pytest
 
-from nonebot.compat import PYDANTIC_V2
+from nonebot.compat import PYDANTIC_V2, LegacyUnionField
 from nonebot.config import DOTENV_TYPE, BaseSettings, SettingsConfig, SettingsError
 
 
@@ -32,6 +32,7 @@ class Example(BaseSettings):
             env_nested_delimiter = "__"
 
     simple: str = ""
+    int_str: Union[int, str] = LegacyUnionField(default="")
     complex: list[int] = Field(default=[1])
     complex_none: Optional[list[int]] = None
     complex_union: Union[int, list[int]] = 1
@@ -62,6 +63,8 @@ def test_config_with_env():
     config = Example(_env_file=(".env", ".env.example"))
     assert config.simple == "simple"
 
+    assert config.int_str == 123
+
     assert config.complex == [1, 2, 3]
 
     assert config.complex_none is None
diff --git a/tests/uv.lock b/tests/uv.lock
new file mode 100644
index 00000000..7518fc90
--- /dev/null
+++ b/tests/uv.lock
@@ -0,0 +1,3 @@
+version = 1
+revision = 3
+requires-python = ">=3.12"