Abstract: During the process of deep cut and fill mining in underground mines, the retention of security pillars is inevitable.The traditional method of delineation of security pillars is obvious.It is considered feasible to demarcate the security pillars with safe depth and rock migration angle.However, calculation method is not clear.With the project of Sanxin gold and copper mine Taohuazui mining area as the engineering background, the safety depth is studied by theoretical calculation and finite element numerical simulation method, and then the security pillars are delineated.The results show that the critical depth of safety is -428.9 m by theoretical calculation.The numerical simulation software FLAC^3D is used to simulate the displacement and stress distribution of roof during the mining process.It is further confirmed that the safety depth is -442 m.Based on the theory of safety depth, the pillar of security is newly delineated.Compared with the traditional method, a large amount of ore is released and the utilization rate of mineral resources is increased.