Abstract: The transient thermal hydraulic characteristics of CPR1000 were analyzed using RELAP5/MOD3.4 code to verify the ability of the adopting water-cooled heat exchanger’s emergency passive heat sink system (WEPHS) for accident mitigation under the condition of feed water line break (FLB) with different break area. The calculation results show that the WEPHS of CPR1000 can supply the water to steam generator immediately and remove the core residual heat after the FLB successfully. With the broken area increasing, the pressure and temperature of the primary loop increase higher, and decrease more sharply with the WEPHS operation. It also shows that the design of WEPHS of CPR1000 is effective.