Field geological investigation shows that the unstable rock block collapse in limestone area is different from that of sandstone area. When disintegration occurs, the disintegration phenomenon is obvious, and the col- lapse disaster often features large areas of damage and may yield a significant air cushion effect. The rock mass structure of unstable rock block in limestone area is the main cause of the above phenomenon. In this paper, the energy equation of a type of unstable rock block system was established based on the fracture model of a type of masonry unstable rock block. The fracture mechanics method and the energy conservation principle were adopt- ed, the criterion of disintegration or failure of rock mass was put forward, and the sudden fracture in the course of sliding of rock block was explained from the angle of energy, and the deflection angle of W2 was predicted to be 34°. The research results of this paper provide a scientific basis for scientific assessment of the stability of unstable rock block, which has great theoretical and practical significance.