Abstract: At present, in-vessel retention (IVR) strategy is widely used in the world to mitigate the consequences of severe accidents. Based on the nuclear power plant accident analysis program SAMPSON developed by the Applied Energy Research Institute of Japan, the debris cooling analysis (DCA) module of SAMPSON program was improved, the stratified model of ceramic layer and metallic layer in molten pool was added, the pre-processing program for interface geometric parameters between 3D Cartesian meshes of melt and 3D surface coordinates of pressure vessel was developed, and the heat transfer relationship of the outer surface of the pressure vessel was improved. The improved program was verified by the IVR under the severe accident of the AP1000 nuclear power plant, and the calculated results were compared with experimental results. The results show that the improved SAMPSON program can effectively simulate and analyze the cooling and retention of melt in the lower head for nuclear power plant under severe accident.