Abstract: In order to evaluate the effectiveness of normal residual heat removal system (RNS) for RCS injection strategy during severe accidents, two typical severe accident sequences were selected and analyzed with MAAP4 code, including direct vessel injection (DVI) line break induced severe accident which contributes most to the core damage frequency, and loss of feed water (LOFW) induced severe accident which is a typical high-pressure core melt accident. Both mitigation effect and negative impact of the RNS injection strategy were evaluated. The results show that RNS injection can effectively terminate core melt progression and keep long-term core cooling in two typical severe accident sequences. However, more hydrogen generates in core during core reflooding stage which may lead to hydrogen hazard in the containment. Furthermore, the influence of injection flowrate was discussed with the minimum injection flowrate for long term decay heat removal given by the computational aid documents from the severe accident management guideline. The analysis results indicate that higher flowrate should be preferred for RCS injection strategy during severe accidents if the injection system would be available.