In this paper, the propagation rate of the cracks at different grain sizes with different grain boundary (GB) angles have been simulated. During loading, the crack tip (CT) emits dislocations, which are distributed on the slip plane (SP) in front of the crack, and may accumulate at the GB, during which the crack expands forward. If the conditions are met during unloading, the dislocations will rebound. Dislocation annihilation will occur when distance of dislocation emitting is less than the distance of dislocation rebound. The annihilation of the dislocations have an impact on the next loading. The calculation results show that the crack propagation rate (CPR) is related to both the grain size and the angle of the GB. The CPR increases in larger crystals and in larger angle GBs. And the number of annihilation dislocations and the number and distribution of existing dislocations will obviously affect the next loading step.