Abstract: The existence of goaf is an important factor resulting in mine disaster. It is still an unresolved problem to make sure the roof safe thickness of goaf. In this paper，a new calculation method of goaf roof thickness is put forward based on thin plate theory，which reveals the relationship between the safe thickness of goaf roof，geometry size of roof and mechanical property of ore. On the basis of the theoretical solution，the reason and process of the roof damage is analyzed from the mechanical point of view. A numerical mechanical model of goaf roof is established using the finite element analysis software，ABAQUS. The failure process of goaf roof and its effect factors are considered. The stress，strain，and displacement of the goaf roof are analyzed through numerical simulation. The influence of roof thickness and goaf span on the stress and displacement field are obtained，and the regression curve and equation of the roof safe thickness and goaf span are derived. Finally，compared the theoretical solution with numerical simulation results，Qidashan roof thickness can be determined.

Key words: Safety goaf thickness, Critical span, Plate theory, ABAQUS, Numerical simulation