Hi Guangda,
Thanks for the reply. I have replied to your question in The android SDK "sample" demo app stuck in "preparing meeting" black screen - #4 by carson.zoom
On the other hand, regarding the following that does not exist in another question:
You could specify your own meeting activity in config.xml like https://github.com/zoom/zoom-sdk-android/blob/master/mobilertc-android-studio/example2/src/main/res/values/config.xml#L4, then extends the MeetingActivity in your MyMeetingActivity. Then you could define your own logic in your MyMeetingActivity(Such as override the back button, etc.)
Hope this helps. Thanks!