Abstract: In order to improve the single-stage efficiency of D46-50X4 multistage centrifugal pump, the k-ε turbulence model was employed, and with CFX fluid analysis software the numerical simulation of the internal flow field was carried on to find out the main reason of efficiency loss. Then new guide vane and impeller were designed, the flow fields of the original model and improved model were simulated and analyzed. The results indicate that in the original model of centrifugal pump, the entrances of the impeller and the diffuser section of guide vane have obvious vortex, which seriously reduces the efficiency of the pump. By decreasing the impeller diameter and adjusting the shape line of impeller, the disk friction loss and vortex in impeller flow passage of the original model are improved. Under standard operating conditions, compared with the original model, the single-stage head of the improved model is increased by 4.4 m, and the efficiency is increased by 6.4%, which meets the national standard.