Abstract: AFT Fathom code was used to calculate the low pressure safety injection of AP1000 nuclear power plant in-containment refueling water storage tank (IRWST). The safety injection performance of AP1000 nuclear power plant in the late period of direct vessel injection line (DVI) break accident was analyzed. The results show that the low pressure safety injection capability of AP1000 nuclear power plant meets the requirement of decay heat removing with reasonable margin.