Abstract: Yilong marble stone mines should be mined under certain thickness of safety pillars and weathering layer.However, the security pillar is too thick that will lead to the waste of resources, on the contrary, the safety of mine production can’t be ensured.According to the characteristics of the ore bodies in Yilong stone mine, the reasonable thickness of safe pillar under different span of goaf was analyzed by theoretical calculation and FLAC^3D software.Through comprehensive analysis, the relationship between the various span of the empty area and the reasonable thickness of the roof was obtained.Based on the largest mining span is 36 m in the mine and the thickness of weathering layer is 4.5 m, the reasonable thickness of roof in Yilong mine is recommended as 28.5 m.