Abstract: The closed Brayton cycle is one of the key techniques of the Generation Ⅳ nuclear power system with high temperature gas-cooled reactor (HTR). In order to analyze its dynamic processes, the dynamic models of key components in the system were built. For the compressor, the radial equilibrium model coupled with the prediction of end-wall boundary layer development was used with combined consideration of efficiency and accuracy. The heat exchanger dynamic model was based on hyperbolic conservation law equations considering the temperature-dependency of helium’s thermal physical properties and the fast changes of thermal parameters in the cycle. The components’ models were coupled as the system dynamic model based on the mass conservation and pressure balance. The bypass valve regulation is the major method for quick adjustment of output power in the helium turbine system, and the transient effects are prominent, so that the transients after bypass valve opened in HTR-10GT system were analyzed as examples. With the system dynamic model, the dynamic responses of thermal parameters were calculated and the mechanism of power output decrease was analyzed. It is shown that the system volume affects the speed of dynamic responses, and the bypass valve opening degree determines the power output at the end of the transients. Moreover, the thermal shock phenomenon is likely to occur in the regulation process but it can be relieved with combined operation of two bypass valves at different locations. Finally, it is proved that the core outlet temperature is almost constant, and the accuracy of the reactor model is of almost no effects to the calculation results.