Abstract: In order to analyze the influence of the high temperature and high pressure coolant on reactor coolant pump, the multiple field coupling numerical simulation of nuclear main pump was done by the ANSYS Workbench software, and the distributions of flow field, temperature field and stress field were obtained. The results show that orbicular pump undermines the fluid flow, and internal flow is complex and obvious backflow exists. The temperature of front coverplate is obviously higher than that of the back coverplate, and the average temperature of front coverplate reaches to 530 K, while the back coverplate is only 500 K. Stress concentration appears at the junction between the front coverplate and the blade, and the minimum equivalent stress of the impeller occurs on the hub side of the back coverplate. The maximum stress occurs at the junction between the coverplate and the outlet of the impeller. The constraints between impeller blades at impeller outlet are weaker while the thermal stress and the tension stress produced by the centrifugal force are bigger, which results in the maximum deformation of the impeller at back coverplate to be 0.24 mm.