Fragmentation characteristics of granite in rotary-percussive drilling (RPD) are studied using the distinct element method. We developed a model to investigate the interaction between the rock and a polycrystalline diamond compact (PDC) cutter. The micro contact parameters in the model are calibrated by conducting a series of simulated mechanical tests of the rock. Sensitivity analyses are then conducted to according the drilling performances which are quantified as the penetration displacement, the fragmentation volume and the specific energy, as well as the lateral force and the particle size distribution. Results show that the model can well represent the typical fracture system under indentation of the cutter, the torque fluctuation phenomenon in drilling, and the formation of lateral chips, which verify the reliability of the model. The cutter with a back rake angle of 55 deg and impact frequency of 30 Hz has the best penetration performance in evaluated parameters. Increasing the frequency has a great effect on the rock breaking speed under the coupling effect of impact and cutting in the low frequency range. Considering crushing efficiency, 50 Hz is the recommended impact frequency. This paper provides a useful tool to represent the fragmentation performance of rotary-percussive drilling and sensitivity analyses shed light on the potential ways to improve the performance.