Spatial structure of overlying strata changes and stope roof is deformed because of the coal mining. Fracture morphology of stope roof is closely related to mining field pressure and surface subsidence. Based on the mechanical model of elastic thin plate, the variational method is used to analyze the formation mechanism of roof fracture, and caving law of stope roof is obtained. The calculation results show that: in the initial failure, the roof break appears along the edges of roof in an “O” form, and then the break of “X” form appears in the center of the roof. With the development of fracture, the “O-X” form appears. When the roof is destroyed in cycles, the half “X” form is formed due to the advancing of the work face and the effect of the angle effect.