Abstract: With the use of best estimate computer code RELAP/SCDAPSIM/MOD3.4 of pressure water reactor severe accident, a three-channel along radial and ten-nodal-along axis nuclear reactor severe accident calculation model was established based on a hypothetical small PWR. The severe accident induced by total loss of power supply was studied, and mitigation measure with 300 s continuation of the steam generator auxiliary feedwater was analyzed. The calculation results show that the steam generator auxiliary feedwater plays an important part in delaying core melt progression and mitigating severe accident consequences.