Abstract:The key to mining with pressure is to understand the floor failure mechanism by mining and water pressure. The failure of the floor rock mass depends on floor stress field and distributions of the primary discon- tinuities. First,based on the rock engineering failure criteria,the definition of the floor failure is discussed. Then,according to the in-situ comprehensive observation data,by using two damage variables of the isotropic elastic damage model to describe the primary discontinuities of the floor,the mesoscopic mechanism of the floor failure is studied with displacement back analysis. The presented study provides the foundation to evaluate the floor safety.