Abstract: For the analysis of loss of normal feedwater accident for AP1000, a thermal-hydraulic model was built based on a two-fluid best estimate code, RELAP5. A control system model was built based on Matlab/Simulink code. As the main data for modeling was from AP1000 Design Control Document (AP1000 DCD), which was inadequate and not accurate enough, the accident analysis was qualitative. The calculation results, overall consistent with the same accident in DCD, show that the RELAP5 code has the ability to calculate the natural circulation. PRHRS and CMT are able to remove the core residual heat, decay heat timely and efficiently during the accident to guarantee the safety of the core. In the calculation model, the capability of PRHRS is stronger, which makes the coolant temperature quickly reduced to a lower level, leading to an earlier actuation of CMT, followed by different responses of the plant.