A scientific and reasonable design of safety thickness of roof is the basis for stoping the orebody No.92 which under the overlap zones in Tongkeng Mine. Based on analyzing the stability factors influencing the roof of orebody No.92

the finite element model is set up; and the simulation is conducted repeatedly. Then value of safety thickness of the roof in conformity to various groups of affecting elements is worked out. According to the simulated results

the curves between every factor and safety thickness of roof are drawn. A mathematical forecasting model of safety thickness of roof under the complicated backfilling

which considers effect among most of the factors

is obtained by stepwise regression multianalysis. The results which combine engineering practice with numerical simulation validate the feasibility of the forecasting model.