Abstract: In this paper, the impeller of the reactor coolant pump is analyzed by Computational Fluid Dynamics (CFD) method to obtain its hydraulic performances. According to the CFD analysis results, the impeller cannot match the system requirements and need be optimized because of the lower head under rated flow, the high flow incidence at the inlet of the blade around the region near the hub, and the high net positive suction head (NPSHr). Optimization targets are determined according to the calculation tolerances and engineering experiences. The impeller inlet angle and outlet angle, and the location of the head of blade are determined as variables of the optimization. After calculation and comparison, the final optimization proposal is determined. The analysis results for the optimized impeller show that the flow incidence at the inlet of the blade is decreased obviously, anti-cavitation capability and internal flow conditions in the impeller are better than the original one, and the optimization target is achieved.