The force analysis of the roof strata of underground mined-out areas in Sandaozhuang open-pit mine was made,and based on K.B.Lu peinie theory and some assumed conditions a mechanical model of roof strata of mined-out area was built.The corresponding relation between the safe roof thickness and different spans of mined-out area under safety coefficient is 1.3 was established by using numerical simulation software MATLAB.This has significance for the safety maintenance in production of the open-pit mine.