Abstract: There are many open pits in China threatened by cavities under open pit limit.The key to insure safe mining depends on the thickness of safety isolation layer between cavity and bench surface，and it needs to be solved urgently with the steps of bench blasting.Based on the present situation of Dabaoshan mine which is an open pit with lots of harmful cavities under open pit limit，the thickness of safety isolation layer under was calculated by using different theoretical methods，and the results simulated by 2D numerical method were compared with those theoretically calculated ones.The optimum thickness of safety isolation layer under different cavity span was obtained through comparison.Finally，an actual cavity group with three cavities under open-pit limit was modeled in 3D to calculate its stability.These provide a systematic stability analyzing method of cavity under open pit limit for the mine.

Key words: Open-pit, Cavity, Thickness of safety isolation layer, Stability analysis