AP1000 is the third generation of innovation-typed nuclear power plant and widely adopts passive safety design to increase safety and efficiency. Particularly, a Passive Residual Heat Removal (PRHR) system is used to deal with the accident due to the failure of normal residual heat removal path. AP1000 plant model is established by using mechanism-based code. The plant model includes the Reactor Coolant System (RCS), Engineering Safety Features (ESFs), and part simplified second side system. And the fail of normal feedwater flow accident of AP1000 plant is selected to analyze the accident progression. The transient response, thermal-hydraulic phenomena, and the cooling capacity of the PRHR are focused on. The transfer heat power from PRHR to IRWST and reactor core decay power are compared with each other. The result shows that in the case of failure accident due to normal residual heat removal path, the removal heat power of PRHR Heat eXchanger (PRHR HX) is able to match with the reactor core decay power at the late stage of the accident, meets the requirements of sustainable long-term core cooling, and keeps the reactor system in the safe shutdown situation.