Abstract: The large advanced pressurized water reactors mitigate severe accident consequences and reduce the risk of containment failure through the invessel retention (IVR) strategy. The cavity injection system (CIS) is applied to take out the decay heat in the reactor pressure vessel and coolant the pressure vessel surface during reactor severe accidents, so realize the IVR strategy. In this paper, the severe accident analysis software was used to calculate the accident process, thermal hydraulic behavior, core degradation process, heat transfer behavior of the molten pool and distribution of radioactive fission products under the large lossofcoolant accident (LLOCA) of the large advanced pressurized water reactor, and evaluate the accident mitigation ability of the active CIS. The new correlation of critical heat flux (CHF) was used in this research based on the experimental correlation from Configure Ⅴ of the ULPU2000 test. It is calculated that the heat flux of the lower head’s outer surface is always lower than the CHF 72 hours after the accident, the heat transfer regime of the lower head is under fully developed nucleate boiling heat transfer, and the IVR strategy is regarded as a success. Under the IVR strategy, the mass of hydrogen was descended by eliminating the molten coriumconcrete interaction (MCCI) process, thus reducing the risk of hydrogen combustion or explosion in LLOCA had come down, and reducing the threatening likelihood of containment integrity. The pressure and relative humidity of containment in the late phase of the accident were higher compared to the no CIS situation in the reason of the increasing steam from coolant water in the CIS. In addition, the release and migration behaviors of noble gases, nonvolatile and volatile fission products were analyzed. It is found that more radioactive fission products stay in the main system under the IVR strategy, and the resuspension progress of the vapor radionuclides settled on the wall is eliminated. With higher humidity and temperature of the atmosphere in the containment under the IVR strategy, the radionuclides deposited on the containment wall are washed into the pool which is at the bottom of containment by the great amount of condensed water, reducing the mass of radioactive products at the wall and atmosphere. In conclusion, the IVR strategy can manage the distribution of radionuclides better and reduce the threat of radioactive fission products leaking into the environment.