diff --git a/miles/ray/actor_group.py b/miles/ray/actor_group.py
index 122e768649b..730eb494c20 100644
--- a/miles/ray/actor_group.py
+++ b/miles/ray/actor_group.py
@@ -78,7 +78,7 @@ def _allocate_gpus_for_actor(self, pg, num_gpus_per_actor, wandb_run_id: Optiona
         if self.args.use_routing_replay:
             env_vars["ENABLE_ROUTING_REPLAY"] = "1"
 
-        backend = os.environ.get("MILES_BACKEND", "megatron").lower()
+        backend = self.args.train_backend
         if backend == "megatron":
             from miles.backends.megatron_utils import MegatronTrainRayActor
 
diff --git a/miles/utils/arguments.py b/miles/utils/arguments.py
index ad7c7d9fe5d..c31f3e9c3b8 100644
--- a/miles/utils/arguments.py
+++ b/miles/utils/arguments.py
@@ -1,3 +1,4 @@
+import argparse
 import json
 import os
 from typing import Any, Dict
@@ -88,6 +89,17 @@ def add_cluster_arguments(parser):
 
             return parser
 
+        def add_train_arguments(parser):
+            parser.add_argument(
+                "--train-backend",
+                type=str,
+                choices=["megatron", "fsdp"],
+                default="megatron",
+                help="The backend for training.",
+            )
+
+            return parser
+
         # rollout
         def add_rollout_arguments(parser):
             parser.add_argument(
@@ -928,6 +940,7 @@ def add_ci_arguments(parser):
             parser = add_custom_arguments(parser)
 
         parser = add_cluster_arguments(parser)
+        parser = add_train_arguments(parser)
         parser = add_rollout_arguments(parser)
         parser = add_data_arguments(parser)
         parser = add_eval_arguments(parser)
@@ -966,7 +979,7 @@ def warning_for_unfinished_backend(backend: str):
 def parse_args(add_custom_arguments=None):
     add_miles_arguments = get_miles_extra_args_provider(add_custom_arguments)
 
-    backend = os.environ.get("MILES_BACKEND", "megatron").lower()
+    backend = parse_args_train_backend()
     if backend == "megatron":
         from miles.backends.megatron_utils import parse_args as megatron_parse_args
         from miles.backends.megatron_utils import set_default_megatron_args
@@ -1013,6 +1026,16 @@ def parse_args(add_custom_arguments=None):
     return args
 
 
+def parse_args_train_backend():
+    if os.environ.get("MILES_BACKEND") is not None:
+        raise Exception("`MILES_BACKEND` is deprecated, please use --train-backend directly.")
+
+    parser = argparse.ArgumentParser()
+    get_miles_extra_args_provider()(parser)
+    args_partial, _ = parser.parse_known_args()
+    return args_partial.train_backend
+
+
 def miles_validate_args(args):
     if args.kl_coef != 0 or args.use_kl_loss:
         if not os.path.exists(args.ref_load):