diff --git a/README.md b/README.md
index d299a20..28e764c 100644
--- a/README.md
+++ b/README.md
@@ -906,6 +906,7 @@ The inbox is a special project that cannot be deleted. Get its ID via `await cli
 | `TICKTICK_REDIRECT_URI` | No | OAuth2 redirect URI (default: `http://127.0.0.1:8080/callback`) |
 | `TICKTICK_TIMEOUT` | No | Request timeout in seconds (default: `30`) |
 | `TICKTICK_DEVICE_ID` | No | Device ID for V2 API (auto-generated) |
+| `TICKTICK_DOMAIN` | No | TickTick domain to use (default: `ticktick.com`, alternative: `dida365.com`) |
 
 ---
 
diff --git a/src/ticktick_sdk/constants.py b/src/ticktick_sdk/constants.py
index 991bf88..7828e6c 100644
--- a/src/ticktick_sdk/constants.py
+++ b/src/ticktick_sdk/constants.py
@@ -9,16 +9,21 @@
 
 from enum import IntEnum, StrEnum
 from typing import Literal
+import os
 
 
 # =============================================================================
 # API Configuration
 # =============================================================================
+def _get_ticktick_domain():
+    """Get the TickTick domain from environment or default."""
+    domain = os.environ.get("TICKTICK_DOMAIN")
+    return domain if domain else "ticktick.com"
 
 # Base URLs
-TICKTICK_API_BASE_V1 = "https://api.ticktick.com/open/v1"
-TICKTICK_API_BASE_V2 = "https://api.ticktick.com/api/v2"
-TICKTICK_OAUTH_BASE = "https://ticktick.com/oauth"
+TICKTICK_API_BASE_V1 = f"https://api.{_get_ticktick_domain()}/open/v1"
+TICKTICK_API_BASE_V2 = f"https://api.{_get_ticktick_domain()}/api/v2"
+TICKTICK_OAUTH_BASE = f"https://{_get_ticktick_domain()}/oauth"
 
 # Default timeout in seconds
 DEFAULT_TIMEOUT = 30.0
diff --git a/tests/test_constants.py b/tests/test_constants.py
new file mode 100644
index 0000000..0ece049
--- /dev/null
+++ b/tests/test_constants.py
@@ -0,0 +1,32 @@
+"""
+Tests for environment variable support in constants module.
+"""
+
+import os
+from unittest.mock import patch
+
+
+def test_constants_default_domain():
+    """Test that constants use default domain when no env var is set."""
+    with patch.dict(os.environ, {}, clear=True):
+        from importlib import reload
+        import ticktick_sdk.constants as constants
+
+        reload(constants)
+
+        assert "ticktick.com" in constants.TICKTICK_API_BASE_V1
+        assert "ticktick.com" in constants.TICKTICK_API_BASE_V2
+        assert "ticktick.com" in constants.TICKTICK_OAUTH_BASE
+
+
+def test_constants_use_custom_domain():
+    """Test that constants are updated when domain is changed."""
+    with patch.dict(os.environ, {"TICKTICK_DOMAIN": "dida365.com"}):
+        from importlib import reload
+        import ticktick_sdk.constants as constants
+
+        reload(constants)
+
+        assert "dida365.com" in constants.TICKTICK_API_BASE_V1
+        assert "dida365.com" in constants.TICKTICK_API_BASE_V2
+        assert "dida365.com" in constants.TICKTICK_OAUTH_BASE