Abstract: AP1000 plant model was established, which included the reactor coolant system (RCS), engineering safety features (ESF), and related second side pipes system. Feedwater system pipe break accident of AP1000 plant was selected to analyze the accident progression. The transient response, thermal-hydraulic phenomena and the cooling capacity of the passive residual heat removal (PRHR) system was focused on. And the impact of pipe resistance uncertainty on PRHR system cooling ability was analyzed. The results show that the removal heat power of PRHR system heat exchanger can match with the core decay power at the late stage of the accident, and meets the requirements of sustainable long-term core cooling in case of normal residual heat removal path failure. When PRHR system resistance coefficient increases, PRHR system flow rate and heat transfer power decreases, and PRHR system cooling ability will be affected.