Abstract: The downhole flow field of the pilot and reaming two-stage PDC bit was numerically simulated by using the finite element software FLUENT. And the characteristics of downhole velocity fields and pressure fields of four-blade two-stage PDC bit and six-blade two-stage PDC bit were figured out. The simulation results show that due to the existence of low-velocity retention zone at the inner cogging in the blade of pilot bit, the superficial flow velocity of drilling fluid is lower and the cleaning effect is poorer. In addition, the pressure fluctuates when the drilling fluid returning from the downhole of pilot bit flows through the step of step-type annulus hole. Finally, the simulation results of two kinds of two-stage PDC bits were compared. It is indicated that the downhole hydraulic energy of pilot body and reaming body of four-blade two-stage PDC bit is distributed more evenly and larger axial pressure difference is formed in the pilot space and the reaming space, which is more favorable for the cuttings carrying and returning of drilling fluid. The match between the hole size of pilot bit, the pilot bit and the open area of the reaming bit nozzle has a significant effect on the downhole flow field of two-stage PDC bit.