游戏需要上架googlePlay，但是apk大于100M，需要使用扩展文件。
http://blog.csdn.net/pdw_jsp/article/details/45150061
参考这篇贴。
只要导入插件 https://pan.baidu.com/s/1jILhKfO
修改其中example的代码：

using UnityEngine;
using System.Collections;

public class DownloadObbExample : MonoBehaviour 
{
    private string expPath;
    private string logtxt;
    private bool alreadyLogged = false;
    private string nextScene = "Main1";
    private bool downloadStarted;

    //public Texture2D background;
    //public GUISkin mySkin;

    void log( string t )
    {
        logtxt += t + "\n";
        print("MYLOG " + t);
    }
    void OnGUI()
    {
        //GUI.skin = mySkin;
        //GUI.DrawTexture(new Rect(0,0,background.width,background.height),background);

        if (!GooglePlayDownloader.RunningOnAndroid())
        {
            GUI.Label(new Rect(10, 10, Screen.width-10, 20), "Use GooglePlayDownloader only on Android device!");
            return;
        }

        expPath = GooglePlayDownloader.GetExpansionFilePath();
        if (expPath == null)
        {
            GUI.Label(new Rect(10, 10, Screen.width-10, 20), "External storage is not available!");
        }
        else
        {
            string mainPath = GooglePlayDownloader.GetMainOBBPath(expPath);
            string patchPath = GooglePlayDownloader.GetPatchOBBPath(expPath);
            if( alreadyLogged == false )
            {
                alreadyLogged = true;
                log( "expPath = "  + expPath );
                log( "Main = "  + mainPath );
                log( "Main = " + mainPath.Substring(expPath.Length));

                if (mainPath != null)
                    StartCoroutine(loadLevel());

            }
            //GUI.Label(new Rect(10, 10, Screen.width-10, Screen.height-10), logtxt );

            if (mainPath == null)
            {
                GUI.Label(new Rect(Screen.width-600, Screen.height-230, 430, 60), "The game needs to download 200MB of game content. It's recommanded to use WIFI connexion.");
                if (GUI.Button(new Rect(Screen.width-500, Screen.height-170, 250, 60), "Start Download !"))
                {
                    GooglePlayDownloader.FetchOBB();
                    StartCoroutine(loadLevel());
                }
            }

        }

    }
    protected IEnumerator loadLevel() 
    { 
        string mainPath;
        do
        {
            yield return new WaitForSeconds(0.5f);
            mainPath = GooglePlayDownloader.GetMainOBBPath(expPath);    
            log("waiting mainPath "+mainPath);
        }
        while( mainPath == null);

        if( downloadStarted == false )
        {
            downloadStarted = true;

            string uri = "file://" + mainPath;
            log("downloading " + uri);
            WWW www = WWW.LoadFromCacheOrDownload(uri , 0);     

            // Wait for download to complete
            yield return www;

            if (www.error != null)
            {
                log ("wwww error " + www.error);
            }
            else
            {
                Application.LoadLevel(nextScene);
            }
        }
    }

}

覆盖package中的代码就可以了。
导出的apk,obb在真机上使用的时候，需要先安装apk，然后再将对应obb文件改名为：
main.bundle version code.<包名>.obb
例：main.1.com.company.TestPack.obb
拷贝到Android设备的“/android/obb/com. Demo.ABC/”路径下

Mark：没考虑到sdk相互接入问题