Abstract: The core makeup tank (CMT) is an important part of the passive core cooling system (PXS) of AP1000 nuclear power plant. Typically, while the main pump is on, CMT can not inject into the core even it is actuated. However, CMT’s injection becomes successful under certain accident scenarios. This influences the accident process and is very important to accident analyses. This study analyzed the loss of normal feedwater flow anticipated transient without scram (ATWS) accident using the general system code RELAP5 MOD3.1. The result confirms the conclusion of the Westinghouse system code LOFT4AP2.0.1. The discussion was made to find out the reason of CMT’s successful injection.