Abstract: In order to investigate the stable startup control scheme of the compressor and the system characteristics of the compressor when operating alone in a supercritical CO₂ closed Brayton cycle, the compressor speed-up experiment and corresponding dynamic simulation are conducted on the MW-level experimental system. The experiment reveals that when the working characteristic of the compressor changes, a mutual transition from a supercritical state to a gaseous state occurrs at the compressor inlet. Additionally, valve switching puts the compressor in an unstable condition. The characteristics of the uneven response of the parameters of each part of the system are analyzed. According to the simulation results, the compressor enters the stage of weakening forward flow within 80 seconds of the valve being switched, and then enters the stage of fluctuating reverse pressure within 889 seconds. The outlet negative pressure difference first increases, then decreases, and finally returns to the forward flow. The simulation results and the experiment results are essentially in agreement. The changes in pressure and CO₂ inventory in each part after opening the compressor bypass valve are less than 2.3 percent and 3.6 percent, respectively. Hence, large pressure changes at the compressor outlet are thought to be efficiently avoided by opening the compressor bypass valve or using the turbine bypass valve alone to control the pressure, which is beneficial for the compressor's steady startup.