Based on thermohydraulic analysis program RELAP5 and control system simulation tool Simulink, this paper builds a small reactor simulation model including the primary system, designs the nitrogen pressurizer control system and other control systems, and realizes the coupling of the thermohydraulic model and control system through the NURES platform. On the basis of establishing the thermal model and implementing the control strategy, the steady-state and transient conditions are verified. The results show that the steady-state calculations of the model are in good agreement with the design values, and the model responds well under normal transient conditions, which can accurately simulate the reactor operation characteristics. The sensitivity analysis method is used to determine the pressurizer control system parameters according to the results of load rejection condition, and the response of important parameters of the system is analyzed. The simulation results show that the nitrogen pressure pressurizer system has a good pressure control effect in the transient conditions, and the primary circuit pressure can be effectively controlled in the intense transient condition. The automatic control scheme of the nitrogen pressure pressurizer system is reasonable and effective, and can provide a certain theoretical reference for the application of the nitrogen pressure pressurizer system in the practical engineering of nuclear power plant.