Abstract: According to the characteristics of the multilayer ore mining,stope roof is assumed to be a simply supported beam,combined with rock beam theory,material mechanics theory,rock mechanics knowledge,the equation between the maximum tensile stress of roof and the thickness of the above rock strata is deduced.According to this,it comes to that rock on the roof and its adjacent seam has an effect on the stability of the roof.In addition,this paper obtains the limited span of strata roof with its formula,and determines to put the limited span as the stabilization basis.