Abstract: Based on the microscopic damage mechanics and the dynamic finite element method, the fragmentation process of jointed rock mass under dynamic loading is simulated. In the dynamic analysis model, the viscous boundary is considered to eliminate the influence of the reflected stress waves from the boundary. The numerical results show the joints change the stress wave propagation mode in the rock masses. On one side, the reflected tensile stress on the surface of the joints enhances the fracturing of the rock between the joint and the drill bit. On the other side, the fracturing toward the specimen bottom is restrained due to the fact that the dynamic wave propagation is absorbed by the weak joints. The fragmentation is also influenced by the joint spacing. The rock specimens are more obviously fractured with smaller joint spacing. The inclination of the joints leads to non-symmetric fragmentation at the two sides of the bit and affects the ability of the downward extension of the main crack. The simulation will be helpful for understanding the mechanism of dynamic failure of jointed rock mass and underground engineering.