I found a solution from this topic
http://www.cocos2d-x.org/forums/11/topics/3181?r=40339
but the code it provided doesnot work very well…
b2PolygonShape:Set() function seems to have some problems
Maybe there are somewhere bugs, but I dont know how to fix it
That topic was posted almost 3 years ago! I wonder if there are some better ways now to use box2d in LUA?