Present an optimized system including improving the capacity, configuration and operation of the cooling system for maintaining the integrity of the reactor vessel during severe accident, which can cool the molten corium inside reactor vessel by external vessel cooling by injecting water to cavity, and prevent reactor vessel failure to ensure the retention of molten corium. The system includes active part and passive part. The In Containment Refueling Water Storage Tank (IRWST) and Fire Water Tanks (FWT) are the source of water for active part, the Passive Tank is the source of water for passive part. The system is one of the main mitigation measures during severe accidents in nuclear power plants, the system configuration, operation and capacity is very important, which decide the safety, reliability and economy of plants. Also use the software “FLOWMASTER” to simulate and calculate the system, to fix the main equipment parameters and verify the system satisfying the requirement of flow of severe accidents. Finally resolve the problem of adopting existing low voltage SBO(station black-out) power for active motor-operated equipment and the two pumps starting has interval time, raise the rate of utilization of passive injection by judging the accident and according to the accident process, enhance the ability of dealing with various severe accidents using the cooling system of maintain the integrity of the reactor vessel during severe accident, which lay a solid foundation for improving the ability of maintaining the integrity of the reactor vessel during severe accident in subsequent advanced pressurized water reactor, which has great significance for reliability operation and economic benefit of nuclear power plant.