Abstract: A mathematical model of liquid nitrogen system of a space environmental simulator is introduced and a quasi-stationary control method is developed in this paper. The variables describing the system characteristics include pressure, temperature, height of liquid, flow, etc. The input variables to the system include travel of valve, heating power, pressure of liquid nitrogen in cooler, heat load, etc. For convenience, a single-phase hermetic cycle system is divided into three blocks: cooler block, pump block and cryogenic sink block. The equations describing pressure and temperature of the three blocks are obtained, and then the mathematical model is built. The pressure, temperature and flow at each critical point in the system are obtained by solving the equations. The regulating valves of the liquid nitrogen system are typical nonlinearity elements, so the control of coupling loops is difficult. Two types of quasi-stationary control method of the regulating valves are developed in this paper, namely absolute regulating method and relative regulating method. The first method depends on the measurement of the flow of each pipe. The secondmethod does not require it and can be used in the nonlinearity control of the valves. The result shows that relative regulating method has many advantages: small overshoot, short regulating time and simple algorithm, which are needed for the control of liquid nitrogen system.