Abstract: The system analysis code for small direct Brayton cycle reactor systems was implemented using Matlab/simulink code. The mathematical and physical model was established by modularization idea. Steady-state and transient operating conditions of the reactor system were simulated by changing the operating parameters of the primary equipment including the reactor, turbine, compressor and heat exchanger. The variation curves of the reactor power, inlet and outlet pressures and temperatures were obtained. The results show that the system analysis code can accurately simulate the steady and transient operating characteristics of the gas-cooled reactor system and the parameter response characteristics of the primary equipment. The research result can provide a basis for the design, optimization and safety analysis of small direct Brayton cycle reactor system.