The deep rocks generally present high strength, plasticity and abrasiveness, which greatly limit the efficient drilling of the bit. Particle impact drilling (PID) technology is a new and efficient drilling method, but its rock-breaking mechanism is still unclear. In present study, a numerical model which truly reflects the mineral composition and granule diameter distribution of granite is established based on the particle flow code (PFC) to optimize the particle impact parameters. The law and mechanism of rock breaking by single particle impact, multi-particle impact and auxiliary cutting with different particle diameter, incident angle and incident velocity are studied. The results showed that the crack number and broken volume fluctuate with the change of incident parameters. Microcracks mostly occur in the longitudinal direction of rock. During particle impact, the formation, expansion and coalescence of microcracks are all along the mineral crystal, while the situation is just the opposite in cutting. With particle impact assisted, the cutter can easily penetrate into the rock and the rock presents brittle failure. This research also includes experiments to examine the generation of cracks and chips. To improve the efficiency of auxiliary rock breaking, it is recommended to use particles with a diameter of 3.0 mm and an incident angle of 0°–30°, while increasing the impact velocity. This study deepens the understanding of the mechanism of rock breaking assisted by particle impact, and provides the basis for designing drilling parameters using the PID technology.