The process of rock cutting was simulated by discrete element method software where the reasonable contact model for rock summarized from the micro bond contact experiment was implanted. The macroscopic and microscopic mechanism on the crack propagation of each stage during rock cutting was investigated. The results show that: the process of rock cutting can be divided into three stages, i.e. loading stage, unloading stage and residual leaping broken stage; failures of the bonds mainly exhibit tensile failure and sheartorsion failure. The crack under the tip is mainly caused by sheartorsion failures while oblique crack is mainly caused by tensile failures. Therefore, the tension failure theory was confirmed. The unloading stage is the critical stage of the overall process and the efficiency of rock fragmentation will be improved if the fragmentation behavior of unloading stage is efficiently used.