diff --git a/dpdata/plugins/ase.py b/dpdata/plugins/ase.py
index 48d8ca68..c07e3ca9 100644
--- a/dpdata/plugins/ase.py
+++ b/dpdata/plugins/ase.py
@@ -1,4 +1,4 @@
-from typing import TYPE_CHECKING, Optional, Type
+from typing import TYPE_CHECKING, Optional, Type, Generator, List
 
 import numpy as np
 
@@ -114,7 +114,7 @@ def from_multi_systems(
         step: Optional[int] = None,
         ase_fmt: Optional[str] = None,
         **kwargs,
-    ) -> object:  # generator of "ase.Atoms"
+    ) -> Generator["ase.Atoms"]:
         """Convert a ASE supported file to ASE Atoms.
 
         It will finally be converted to MultiSystems.
@@ -142,7 +142,7 @@ def from_multi_systems(
         frames = ase.io.read(file_name, format=ase_fmt, index=slice(begin, end, step))
         yield from frames
 
-    def to_system(self, data, **kwargs):
+    def to_system(self, data, **kwargs) -> List["ase.Atoms"]:
         """Convert System to ASE Atom obj."""
         from ase import Atoms
 
@@ -160,7 +160,7 @@ def to_system(self, data, **kwargs):
 
         return structures
 
-    def to_labeled_system(self, data, *args, **kwargs):
+    def to_labeled_system(self, data, *args, **kwargs) -> List["ase.Atoms"]:
         """Convert System to ASE Atoms object."""
         from ase import Atoms
         from ase.calculators.singlepoint import SinglePointCalculator
@@ -298,20 +298,35 @@ def from_labeled_system(
 
         return dict_frames
 
-    def to_system(self, data, **kwargs):
-        """Convert System to ASE Atoms object."""
+    def to_system(self,
+                  data,
+                  file_name: str = "confs.traj",
+                  **kwargs) -> None:
+        """Convert System to ASE Atoms object.
+
+        Parameters
+        ----------
+        file_name : str
+            path to file
+        """
         list_atoms = ASEStructureFormat().to_system(data, **kwargs)
-        file_name = kwargs.get("file_name", "conf.traj")
-        traj = Trajectory(file_name, "a")
+        traj = Trajectory(file_name, 'a')
         _ = [traj.write(atom) for atom in list_atoms]
         traj.close()
         return
 
-    def to_labeled_system(self, data, *args, **kwargs):
-        """Convert System to ASE Atoms object."""
+    def to_labeled_system(self,
+                          data,
+                          file_name: str = "labeled_confs.traj",
+                          *args, **kwargs) -> None:
+        """Convert System to ASE Atoms object.
+        Parameters
+        ----------
+        file_name : str
+            path to file
+        """
         list_atoms = ASEStructureFormat().to_labeled_system(data, *args, **kwargs)
-        file_name = kwargs.get("file_name", "labeled_conf.traj")
-        traj = Trajectory(file_name, "a")
+        traj = Trajectory(file_name, 'a')
         _ = [traj.write(atom) for atom in list_atoms]
         traj.close()
         return