Abstract: The normal residual heat removal system（RNS） is one of minority active systems of passive nuclear power plant. The water makeup by RNS injection under loss of coolant accident（LOCA） could supply defense in depth function for the passive systems. Based on thermal hydraulic analysis, the permit time window for RNS injection was calculated under various conditions. The RNS injection was modeled according to the emergency operation procedure and human reliability analysis to evaluate the risk influence of the plant. It indicates that there was sufficient time for operator to achieve the defense in depth function by RNS injection under assumed medium and small LOCA. The plant risk would be obviously increased without RNS injection. While under some rarely condition, such as LOCA plus safety systems failure, the time window for RNS injection is short but the influence to the plant risk is negligible even without RNS injection. RNS injection function takes a big influence to the plant risk, it is recommended to pay attention to the operator training in the procedure, which would benefit to the safety of the plant.