Abstract: The reactor system response of the mixed neutron energy spectrum of supercritical water cooled reactor (SCWR-M) under two types of reactivity initiated accidents (inadvertent control rod withdrawal and rod ejection) was studied. Firstly, applying the modified system code RELAP5 which is available for supercritical conditions, a model of SCWR-M system was established. Then, in the process of the accident under operation conditions, some important parameters, such as power, flow rate and cladding temperature were calculated and analyzed. Finally, analysis of parameters, such as worth and withdrawal speed of the control rod and negative feedback coefficient of the core was carried out. The results show that under the design condition, SCWR-M can effectively remove the decay heat from the core to ensure the integrity of the fuel rods. In addition, reactivity parameters have little influence on the safety under the control rod withdrawal accident. But they have strong effect on the peak cladding temperature under the control rod ejection accident. The safety margin will be greatly reduced when too conservative parameters are taken into the safety analysis.