Abstract: As for the complex rear wear-ring return-flow system, a method is proposed to simplify bearing model. The method was applied and based on the simplified model, the flowing of three-dimensional turbulent in a high-speed pump (38500r/min) was computed by using the time-averaged Navier-Stokes equation and the SIMPLEC algorithm. The result shows that this method is feasible in engineering practice. On this basic, the models with different rear wear-ring clearance were simulated. The external characteristic and the fluid flow of the pump under all circumstances were analyzed. Research shows that the rear wear-ring influences the external characteristic of the pump obviously. The leakage of rear wear-ring is great influenced by the size of wear-ring, and is roughly linear relationship with the clearance size. The increase of clearance of rear wear-ring can improve the flow state in rear cavity and decrease the vortex region, and smooth the flow.