The paper describes the rock breaking test of the PDC bit in mudstone under different downhole pressure conditions(from 0 to 5000m). The results indicate that the.cutting force needed for breaking the rock increases with depth (confining pressure); meanwhile, the cutting efficiency is influenced to some degree by the cutter shape of the bit and the bit configuration. Therefor, the cutter shape and the bit configuration should be designed according to the well depth and the rock characteristics so as to increase the penetration rate.