Abstract: HPR1000 is designed with cavity injection system (CIS) which can realize in-vessel retention during severe accidents. CIS includes active and passive serials, among which passive CIS is designed to mitigate severe accident induced by station black-out (SBO). The accident mitigation capability of passive CIS was evaluated in this paper. A calculation code for molten pool heat transfer in lower head was developed and validated. Severe accidents initiated by SBO and SBO plus primary loop break with different break sizes were calculated by MAAP code, the maximum heat flux of pressure vessel outer surface under different severe accident sequences was calculated together with the validated code, and then the validity of passive CIS was evaluated for the above accident sequences. The results indicate that passive CIS can mitigate severe accidents initiated by SBO or SBO plus primary loop break with equivalent break size smaller than 60 mm and achieve the IVR strategy. The results can be used in severe accident management for HPR1000.