Finally found out the problem is due to some hidden entity in the autocad file that is very far away that ruin the display buffer. Look like the display buffer have a distant limitation??
The model was model from an autocad dwg file. when the model was bought into formz, some very distant dwg entity are not deleted.
I just pick the model from the sketchup file and copy and paste to another file and save it as Sketch up 6 file. open with form Z 6.62 and export to fact by layers solved the problem. Thank for all who responded:)