Abstract: Space power system based on fission nuclear reactor has advantages of large electric power, high specific power and long-time running compared with solar energy and chemical fuel battery, and the space nuclear reactor is promising to be widely applied in space exploration. Based on high temperature gas-cooled reactor technology, a design case of space nuclear reactor power system was proposed, where He-Xe mixture was adopted as the coolant of reactor and the working fluid of closed Brayton cycle. The designed space nuclear reactor is a small prismatic high temperature gas-cooled reactor employing the coated particle fuel, and the thermal power is 5 MW. Monte Carlo method was employed for neutronic analysis. The calculated results show that the designed reactor is capable of achieving 10 years of full power operation, having negative reactivity temperature coefficient, and remaining subcritical in submersion accident.