Abstract: In order to deeply study the fracture of rock mass in deep mines with high stress,high water pressure and strong mining disturbance,cracks of floor rock mass after mining were classified into shear crack,layer crack and vertical opening crack according to their formation sites and mechanisms.Simultaneously,the initiation criterions of these three cracks were calculated by theoretical analysis.Through the monitoring results of failure floor at No.1237 working face in Zhaogezhuang Coal Mine by apparatus with double-ended resisting water and measuring leakage,the crack evolution law of deep floor was investigated.The results indicated that the deeper mining,the deeper failure floor；and the preexisting cracks of floor rock mass after mining presents the change process of close-open-close,and initiation-close to the new cracks of rock mass；the initial position and development of failure floor rock mass are associated with some factors such as rock lithology,stress,which validated the accuracy of crack initiation criterions to shear crack,layer crack and vertical opening crack presented in floor rock mass.