Abstract: The numerical analysis was performed on temperature distribution of the outlet nozzles in main vessel of SCWR (supercritical water reactor) by using computational fluid dynamics software CFX. The thermal sleeve between the outlet nozzle and the main vessel was designed to avoid direct connection between them. Thus the temperature gradient of the main vessel closed to the outlet nozzle was decreased, and the safety of the reactor was enhanced. The result shows that the geometrical parameters, such as the diameter and the length of the thermal sleeve, have a definite influence on the temperature gradient of the thermal sleeve connected with the outlet nozzle.