Abstract: Best estimate plus uncertainty (BEPU) method is widely used in design basis accident analysis in nuclear power plant nowadays. Considering complicated severe accident phenomenon and high uncertainty, the BEPU method is seldomly used in severe accident. Core exit temperature (CET) is an important monitoring parameter for safe operation of nuclear power plant (NPP). In this paper, VVER1000 pressurized water reactor nuclear power plant was taken as the research object, the BEPU method was used to study the uncertainty of CET corresponding to cladding rupture under severe accident initialed by large break loss of coolant accident (LBLOCA), and the sensitivity of the input parameters was analyzed. The results show that the unilateral tolerance limit (95/95) of CET corresponding to gas release is 430.85 ℃. The CET is more sensitive to the factor of decay power and the thickness of cladding.