Abstract: The closed Brayton cycle is commonly adopted as the dynamic thermoelectric conversion in megawatt-class nuclear electric propulsion (NEP) system, which characterizes as simple structure and high efficiency. The Brayton cycle in megawatt-class NEP system was analyzed in this paper, main contents are shown as follows: The thermal and physical properties of helium, nitrogen, carbon dioxide, xenon and their mixtures were compared. What’s more, the heat transfer coefficient, pressure loss and the stage number of turbo-machines in the gas turbine cycle in megawatt-class NEP system were analyzed. The performance of direct gas Brayton cycle in megawatt-class NEP system coupled with inter-cooling and pre-cooling process was investigated. The thermal efficiencies of gas turbine in megawatt-class NEP system with different gases as the working fluid were compared. Then, the influence of different parameters on cycle efficiency was analyzed. This research can be functioned as the reference for working fluid choice of gas turbine cycle and the fundamental research of design of megawatt-class NEP system. It also makes a good preparation for transient performance study of Brayton cycle in megawatt-class NEP system.