Abstract: The CFD analysis and experimental results show that guide vane has a significant influence on the hydraulic performance of nuclear main pump. In order to improve the efficiency of nuclear main pump, the definition of diffusivity of the guide vane was proposed based on the optimal specific area. Three factors and two levels were selected to control the diffusivity. Based on the orthogonal test method, the optimal matching scheme of guide vane parameters was obtained. The research results show that under the condition that the specific area of the impeller and the guide vane is constant, the effect of guide vane diffusivity on the performance of upstream impeller is not significant, but is significant on the hydraulic performance of guide vane and the downstream spiral case. With the increase of the guide vane diffusivity from zero, the pump efficiency increases firstly and then decreases; while the diffusivity is 0.025, the pump efficiency is the highest, and the hydraulic loss of guide vane and volute is minimum, at the same time the load distribution of guide vane is reasonable. The method to improve the hydraulic efficiency by adjusting the guide vane diffusivity will provide a theoretical reference for the hydraulic design of the nuclear main pump.