Abstract: The transient characteristic of the simplified system for CPR1000 under the station blackout (SBO) accident was simulated by use of RELAP5/MOD3.2 code, and the capability of mitigating accident results after auxiliary feedwater (AFW) system operating was also analyzed. The calculation results show that: As a result of the loss of the main water supply, the average temperature and the pressure of the primary system reach their maximum in 6 s after SBO accident; the increasing of temperature and pressure caused by the loss of heat sink is effectively mitigated in about 200 s after the input of auxiliary feed water, and this will win more time for the recovery of AC power and the input of residual heat removal system.