Abstract: The reference plant was a typical three-loop PWR of generationⅡ, based on the best estimate computer code RELAP/SCDAPSIM, the failure principle of reactor pressure vessel in severe accident induced by station blackout was analysed. The calculation results show that RELAP/SCDAPSIM program of the COUPLE two-dimensional finite element model can predict the detailed molten material behavior characteristic in pressure vessel, and the lower head failure time and failure position are in good agreement with existing experimental results.