AssetBundle LoadAsset questions in android

Hi everyone,I use LoadAsset Functions to load my asset "sysm.jpg",in unity editor work find,but it can't work in android.this is my building AssetBundle code.

 BuildPipeline.BuildAssetBundles ("Assets/AssetBundles", BuildAssetBundleOptions.None,BuildTarget.Android);

and this is my C# code.

     IEnumerator DownloadANdCache (){
         while (!Caching.ready)
             yield return null;
 
         Debug.Log (BundleURL);
         using (www = WWW.LoadFromCacheOrDownload (BundleURL, version)) {
             while (!www.isDone) {
                 Debug.Log(www.progress);
                 float jdt = www.progress*100;
                 loading_Text.GetComponent<UnityEngine.UI.Text> ().text = (jdt.ToString("f1") + "%");
 
                 loading_Jdt.GetComponent<UnityEngine.UI.Image>().fillAmount = www.progress;
                 yield return null;
             }
             //yield return www;
 
             if (www.error != null) { //some error
                 throw new Exception ("error:" + www.error);
                 Application.Quit ();
             }
 
             AssetBundle bundle = www.assetBundle;
             UnityEngine.Texture2D temp_cx = (bundle.LoadAsset ("sysm.jpg")) as UnityEngine.Texture2D;
             L_Obj [0].GetComponent<UnityEngine.UI.Image> ().sprite = Sprite.Create(temp_cx,new Rect(0,0,temp_cx.width,temp_cx.height),new Vector2(0.5f,0.5f));
         }
     }

My English is not so good,i hope say it clearly. Thank you very much.