diff --git a/news/215.bugfix b/news/215.bugfix
new file mode 100644
index 0000000000000000000000000000000000000000..93a3fb65db22204e65d23f173c7d2d4f31b5cae1
--- /dev/null
+++ b/news/215.bugfix
@@ -0,0 +1 @@
+Fix a bug of `PYTHONPATH` manipulation under Windows platform.
diff --git a/pdm/cli/actions.py b/pdm/cli/actions.py
index 041db9b5ca9dab0eb1c41767911129f3ff66232b..84adba25d906837188991341dd2c4e63c2186f7b 100644
--- a/pdm/cli/actions.py
+++ b/pdm/cli/actions.py
@@ -1,7 +1,6 @@
 import json
 import os
 import shutil
-import sys
 from pathlib import Path
 from typing import Dict, Iterable, List, Optional, Sequence
 
@@ -31,9 +30,7 @@ from pdm.project import Project
 from pdm.resolver import resolve
 from pdm.utils import get_python_version, setdefault
 
-PEP582_PATH = os.path.join(
-    os.path.dirname(sys.modules[__name__.split(".")[0]].__file__), "pep582"
-)
+PEP582_PATH = os.path.join(os.path.dirname(os.path.dirname(__file__)), "pep582")
 
 
 def do_lock(
diff --git a/pdm/cli/utils.py b/pdm/cli/utils.py
index 05bd82198dbd5e13e38f4f341624e10a8eeb9903..16b6dc68868634bf51e4dae5e3e0a376b813ff32 100644
--- a/pdm/cli/utils.py
+++ b/pdm/cli/utils.py
@@ -424,7 +424,7 @@ def set_env_in_reg(env_name: str, value: str) -> None:
                     return
             except FileNotFoundError:
                 old_value, type_ = "", winreg.REG_EXPAND_SZ
-            new_value = ";".join(old_value, value) if old_value else value
+            new_value = os.pathsep.join([old_value, value]) if old_value else value
             try:
                 winreg.SetValueEx(env_key, env_name, 0, type_, new_value)
             except PermissionError: