Thank you very much!
I have set the preference like this picture.
I set the ceiling filtering height to 1m,it works,those point higher than 1m disappear.
But I set the ground filtering height to -0.7m,It didn't work,the ground point didn't appear.
So I think :
Maybe the rtabmap.db has no ground point?
Maybe I should set some parameters in rtabmap.launch to generate(or save) ground pointcloud?