Abstract: In the severe accident, the core melt with high temperature is redistributed in the pressure vessel and exerts a thermal load on the pressure vessel wall, which may lead to the failure of pressure vessel. Based on the Fortran90 language, in-vessel melt retention (IVR) analysis code IVRASA-ELLIP in severe accident of ellipsoidal lower head was developed, which was used to analyze the heat transfer and IVR of the pressure vessel with the ellipsoidal lower head under the severe accident. IVRASA-ELLIP was used to calculate the heat transfer of the VVER-1000, and the results of the wall heat flux, the oxide crust thickness and the wall thickness of the pressure vessel were obtained. The results of heat transfer of AP1000 were obtained through IVRASA code, which were compared with the results of VVER-1000. The results show that with the same initial parameters, the wall heat flux of the ellipsoidal lower head is smaller than that of the spherical lower head, the amount of ablation of the pressure vessel wall with the ellipsoidal lower head is smaller than that with the spherical lower head, and the oxide crust thickness in the pressure vessel with the ellipsoidal lower head is thicker than that with the spherical lower head.