Abstract: The space nuclear thermionic reactor is one of the designs of space nuclear reactor which has been successfully launched into orbit. Although it is of high technical maturity, the complex thermal environment in space is of great challenge to the normal start-up of space nuclear thermionic reactors. When the liquid metal in thermionic reactors is freezing, it will seriously affect the heat discharge in the reactor core, which may lead to severe core disassembly. In this paper, to prevent coolant condensation, an insulation strategy for thermionic reactors in near-earth orbit was designed. It consists of a heat shield and an electric heater. The models of the thermal environment were developed including the earth’s infrared heat flux, the earth’s reflect heat flux and the solar heat flux. Besides, the heat shield model was developed by its heat equilibrium. The space nuclear thermionic reactor system code TASTIN was used to model the system. TASTIN was a modular modeling analysis code for transient thermionic systems. It contains models of pumps, radiators, and thermionic fuel elements. By dividing the control volume of each part, conservation equations about state variables were established. They were solved by Gear or Adams method. Thus, the strategies with or without the heat shield were simulated and analyzed. The strategies containing a heater were fully discussed. The results show that under the influence of the heat flux on orbit, the freezing of the system coolant occurs after 2.84 h without a heat shield and electric heater, and additional antifreezing strategies need to be added. After the 650 W electric heater is introduced without a heat shield, the temperature oscillation of the system coolant is stable at 278.6 K, which can ensure that the coolant will not freeze for a long time. The freezing time of coolant in the system with a heat shield occurs for more than twice as long as that under uncovered working conditions. By adding a 650 W heater, the minimum temperature of the coolant is close to 302 0 K, which prevents the freezing of the system coolant. The strategy with a heat shield and heater improves the heating efficiency and ensures that the coolant freezing time is prolonged by at least 4.95 h in case of failure of the heat shield or heater. The study provides valuable theoretical support for the design and analysis of the anti-freezing strategy with a heat shield and heater. The strategy has a high safety margin and can ensure the normal operation of the system.