Abstract: Based on an integral system analysis computer code, a severe accident sequence induced by loss of feed water (LOFW) plus failure of auxiliary feed water, with external reactor vessel cooling (ERVC) measure, was calculated for 900 MWe nuclear power plant. The validity of the measure for mitigating core’s melting progression, keeping the integrity of the vessel was analyzed, and three effect factors of the measure such as velocity of water injecting, water level in cavity and different injection time, were also analyzed. The results indicate that ERVC with enough mass of water, appropriate velocity of water injecting and water level in cavity, can provide adequate cooling capacity for vessel lower head, and the integrity of vessel can be retained. ERVC in different injection time with appropriate velocity of water injecting, the integrity of vessel can also be retained although the injection time is delayed.