Abstract: Based on the thermionic element working environment, the necessity of gas exhausting was analyzed for thermionic element of TOPAZ-2 space nuclear reactor power. According to the structure feature of TOPAZ-2 space nuclear reactor power, the helium cavity, the emitter and collector of thermionic element, the cesium cavity and the coolant inner cladding were selected as the main gas exhausting components. The working temperatures of the reactor core vessel and thermionic element at rated condition were calculated with CFD and specific programs. Combined with the exhausting requirement of these components, the gas exhausting conditions were decided. A two-dimensional thermal transfer program was developed, which was used for gas exhausting calculation of the emitter and collector of thermionic element. The program was validated by comparing the calculation results with the experiment data of single thermionic element gas exhausting. The gas exhausting power of the emitter and collector of thermionic element was calculated with the program. Integrated with component gas exhausting condition, requirement of gas exhausting environment and calculation result of gas exhausting, the gas exhausting scheme of TOPAZ-2 space nuclear reactor power converter was given.