Abstract: Taking a 1000 MW PWR as an example, a two-dimensional polar coordinate thermal model was used to analyze the coupling heat transfer among the wall surface of RPV, the two-layer melting core pool and the outer water chamber. The transient 2D temperature and ablation of the bottom head wall surface were calculated. At the same time, the finite element analysis program was used to calculate the thermal stress and strain caused by the transient temperature field and ablation of the wall of the lower head. The thermal stress / strain condition was used to analyze the structural integrity of the PWR RPV lower head in the pressure vessel under the In-vessel retention via external reactor vessel cooling (IVR-ERVC). The calculation results show that:①The wall of the lower head began to melt at 200 s after the core collapsed, and the thinnest thickness decreased linearly. After 3000 s, the molten zone along the inner wall of the head formed a lancet shape distribution. ②The endothermic heat flux of the inner surface of the lower head was larger than that of the external surface, and the heat flux reached the maximum value at the interface between the two layers of the molten pool. ③The strain of RPV lower head increased sharply in the period from 0 s to 400 s and then keeps unchanged. The equivalent stress locally concentrated on the inner wall of the lower head at 400 s. After 400 s, the inner wall thermal stress decreased gradually, the shape variable increased, and the integrity of the lower head could be guaranteed. ④After 2000 s, the stress concentration was generated on the inner and outer wall at the ablation of the RPV lower head. The stress value of the inner and outer walls of the lower head was greater than that of allowable stress. The RPV lower head may fail at any time after 2000 s and may be broken at the edge of the ablation area.