Abstract: TOPAZ-Ⅱ space nuclear reactor power supply adopts thermionic static thermoelectric conversion mode, the output power of the system is 5 kW, and its service life can reach 3 years. It is one of the most advanced space nuclear power supply systems that can be rapidly engineered at present. However, the temperature coefficient of moderator in TOPAZ-Ⅱ reactor is a large positive value, which leads to the positive temperature coefficient of the whole reactor, which puts forward high requirements on the reliability of the reactivity control system, so it is necessary to study its mechanism in detail. A three-dimensional accurate MCNP model of TOPAZ-Ⅱ reactor core was established, and the causes of the positive temperature effect of moderator in TOPAZ-Ⅱ reactor were analyzed in detail from three aspects: The change of neutron spectrum, the change of neutron equilibrium relationship and the change of neutron cycle factors. It is found that the positive temperature effect of moderator is the result of the joint action of three factors. First of all, TOPAZ-Ⅱ adopts zirconium hydride solid moderator. Secondly, unlike TRIGA, TOPAZ-Ⅱ adopts the heterogeneous arrangement of rod-shaped fuel inserted in moderator matrix. Finally, TOPAZ-Ⅱ uses highly enriched uranium with ²³⁵U enriched to 96%, and the fuel pellets are thick. Under the synergistic effect of these three factors, the thermal neutron utilization factor of fuel increases obviously after the temperature of moderator increases, which leads to the positive temperature effect. From the point of reducing the heterogeneity in the reactor core, a verification example was established, and it verifies the correctness of the analysis. On this basis, a method to reduce the positive temperature effect of moderator was found, which provides a guiding direction for the optimization design of thermionic reactors in the future.