Abstract: High temperature gas cooled reactor combined with closed cycle magnetohydrodynamic power generation (CCMHD) is a promising technology for space applications. It can meet the requirements in space tasks for high power and high efficiency. The system mainly contains the nuclear reactor, the MHD generator, the regenerator, the compressors and the radiator. Helium is adopted as both the coolant and the power generation medium for its good ionization properties and chemical inertness. A set of 100 MWth space magnetohydrodynamic nuclear power system circulation scheme was proposed in other literatures. In this paper, thermodynamic models and mass models of this system were established dependently, and efficiency analysis along with specific mass analysis was conducted afterwards on the basis of verifying the accuracy of the program. Specifically, the developed analysis program was used, by means of selecting the same design parameters of the system cycle scheme for thermodynamic calculation and specific mass calculation, to compare with the literature results. Besides, the effects of the radiator temperature, the reactor outlet temperature, the number of compressors and the regenerator efficiency were also discussed for the use of parametric optimization. The research shows that increasing the temperature of the reactor outlet, decreasing the minimum cycle temperature, and increasing the number of compressor stages are all beneficial to improve the maximum cycle efficiency, and also increase the corresponding optimal enthalpy extraction rate and optimal pressure ratio. Moreover, increasing the regenerator efficiency is beneficial to improve the maximum cycle efficiency. However, it also reduces the corresponding optimal enthalpy extraction rate and optimal pressure ratio at the same time. After comprehensively considering many factors such as cycle efficiency, system specific mass, radiator area, system complexity, existing manufacturing capacity and technical level, a set of design parameters suitable for 1 MWth thermal power were finally given. The reactor outlet temperature was chosen as 1 800 K, the minimum cycle temperature was chosen to be 300 K, the number of compressor stages was selected as 3 for the system design parameter, and 0.93 was chosen to be the regenerator efficiency. The results show that the cycle efficiency of the system is 46.15%, along with a total mass of 4 375 kg. Besides, the specific mass is 9.48 kg/kWe, and the radiator area is 1 302.7 m², which shows great competitiveness in future space applications. Finally, this research also provides system-level optimization parameters for the subsequent thermal design of the reactor.