Abstract: The complex cosmic thermal environment is a great challenge to the orbit injection of space reactors. Improper design will lead to liquid metal condensation and hinder the safe startup of reactors. Therefore, for the safe startup of liquid metal cooled space reactor, the liquid metal in the system must be insulated to prevent condensation on orbit. In this paper, the onorbit insulation strategy of space thermionic reactor was proposed and a model was established for calculation and verification. In order to simulate the influence of space thermal environment and verify the insulation strategy, this paper established a improved TOPAZ Ⅱ space thermionic reactor model based on the transient analysis code of space thermionic reactor system, TASTIN. The calculation model was divided into several control volumes. For each control volume, the governing equations were discretized based on the finite volume method (FVM), and the transient equations were solved by Gear algorithm. Moreover, the radiator heat transfer model (TASTIN-RAD) and the radiant heat transfer model of the heat shield (TASTIN-SHIELD) were developed for calculation of thermal environment (mainly solar heat flux) and different heat shields respectively. Then the spin and nonspin states of the space reactor on orbit were calculated with considering the solar heat flux. The results show that the spin state is better than the nonspin state in the first flight cycle, while in the subsequent flight cycle, the nonspin state is better than the spin state. After 24 hours’ flight of nonspin in orbit, the temperature of fluid is close to the freezing point and additional thermal insulation measures shall be taken. Thus, different insulation strategies including open and closed heat shields are calculated and analyzed in nonspin state for 24 hours flight. By adding an open heat shield, the solar heat flux has a significant heating effect on the fluid. The minimum temperature of the fluid after 24 hours of flight is 295 K, which can ensure that the fluid does not solidify. Under the closed heat shield scheme, the top of the heat shield has a special absorptionemission ratio, which can heat the system circuit. After 24 hours orbit flight, the minimum temperature of the circuit fluid reaches 303 K. According to the calculation results, only depending on the flight attitude and the heating effect of solar heat flux is not enough to prevent the condensation of liquid metal working medium, so additional insulation measures must be added. Both insulation schemes proposed by this paper effectively ensure the long-term heat dissipation of the system and have high safety margin. The difference is that the closed shield strategy can make the fluid temperature rise slowly, while it decreasing in the open shield strategy, which means the close shield strategy has absolute security. The calculation results of this paper provide a basis for the insulation scheme design of space thermionic reactor.