Abstract: The low pressure injection performance of normal residual heat removal system (RNS) in AP1000 nuclear power plant was analyzed. It is conservatively assumed that under low pressure injection condition, the pressure of reactor coolant system and the minimum injection flow required to prevent the water level drop of core make up tank remain constant. Combined with the layout of RNS low pressure injection pipes and the performance curve of RNS pumps, the low pressure injection performances of RNS in suction from the in-containment refueling water storage tank and cask loading pit were analyzed, respectively. The injection flow and the injection last time are two parameters to evaluate the performance of RNS. Through the analysis, not only the reliability of RNS’s low pressure injection function is verified, but also the actual injection capacity of the two water-sources is quantitatively given.