In-vessel retention (IVR) strategy is a solution widely adopted by Generation Ⅲ reactors to deal with the corium coolability problem by flooding the reactor pit (cavity) during a serve accident of pressurized water reactor (PWR). The stratification of different melted core internals will influence the temperature field and velocity field of the corium pool, thereby affecting the distribution of heat load on the RPV steel shell. In the current safety analysis of PWR, the success criteria for IVR strategy is the local heat flux imposed from the corium pool to the reactor pressure vessel (RPV) wall should not exceed the critical heat flux (CHF) of boiling on the lower head. CFD model was built to simulate multiform dynamic stratification processes of core melt from a mixed state to three layers of light metal, heavy metal and oxide, provides a basis for the further study of thermal hydraulic phenomenons in the corium pool and the integrity of RPV lower head. The simulation results shows that, with the good cooling condition on the outer wall of RPV, the thermal loads on the lower head is significantly less than the corresponding CHF limits under different stratification process, and the integrity of lower head can be maintained during a severe accident.