Abstract: The radiation shielding of space nuclear reactors can reduce the radiation of neutron and gamma photon fluxes to instruments and equipments. It is one of the key components of the power supply of space nuclear reactors. The temperature is an important factor affecting the performance of radiation shielding. The Fluent software was used to numerically simulate the heat transfer behavior of the TOPAZ-Ⅱ space nuclear reactor power supply radiation shield in a vacuum environment. The optimization measures were proposed. And the influence of the optimization measures on the temperature field and its temperature distribution characteristics were revealed. The research results show that the low-emissivity coating sprayed on the surface of the coolant pipe and its pipe groove has the effect of uniform radiation shielding the temperature field and reducing its temperature peak. The optimal parameters are the emittance of the coating on the pipe surface between 0.1 and 0.3, and the emittance of the coating on the pipe groove around 0.1. And the effect of spraying low emissivity coating on the pipe surface is better than that of the pipe groove. In addition, a single layer of vacuum heat shield with surface emissivity of 0.04-0.07 can be added between the coolant pipes and grooves to optimize the radiation shielding temperature field and peak temperature.