In this paper, a dynamic model of a 5MW small modular supercritical CO2 Brayton-cycle reactor system is established based on Modelica language, and the control rod ejection accident simulation with different reactivity is carried out. This paper takes a 5MW small modular supercritical CO2 Brayton cycle reactor system as the research object. Based on the modular modeling idea, the dynamic model of the system including reactor, turbine, compressor, regenerator, cooler, and other main equipment is built, and the reactor power control system is based on the PID control principle is designed. The results show that the system has certain inherent safety, and the designed control system can coordinate the main equipment under accident conditions, maintain the important parameters such as reactor core fuel temperature, coolant temperature, and reactor pressure within the limit, and control the system flow, permeability and compressor rotor speed within a reasonable range. The work of this paper provides a certain reference for the selection of control strategy and safety system design of the new supercritical CO2 reactor system.