Abstract: Nuclear reactor power has the characteristics of long service life and all-weather work, which can be used as power for planetary surface and other deep space exploration missions. In view of the criticality safety problem of nuclear reactor back to the ground during launch, the criticality safety analysis requirements, analysis assumptions and models for planetary surface nuclear reactor were proposed. The criticality safety characteristics of reactors and the criticality safety measures taken were calculated and analyzed. The calculation results show that the effective multiplication factor of nuclear reactor under different hypothesis falling environments is less than 0.98, which meets the criticality safety requirements. The use of Mo-14%Re alloy structural material, relatively thick BeO reflector layer and Gd₂O₃ coating on the reflector layer cladding and core periphery can effectively reduce the criticality accident risk and ensure that the reactor is in a subcritical state.