Abstract: Under core melt accident of pressurized water reactor, the hot spot of bottom head causes local overheating of pressure vessel. This severe accident leads to critical heat flux. The hot spot of bottom head was calculated under core melt accident by using FLUENT software and the weaknesses of bottom head from the view of flow and heat transfer were predicted. The results show that when the reactor core melts, there are two weakest locations in bottom head of pressure vessel, which are bottom of bottom head and border of oxidized shell and pressure vessel. On the border of oxidized shell and pressure vessel, variety reasons cause critical heat flux, so border of oxidized shell and pressure vessel suffers severe accident. The melt time of pressure vessel wall can delay by setting the extension tube and additional cooling water.