After the Fukushima nuclear accident, many countries and international organizations began to explore and establish new nuclear safety objectives and nuclear safety requirements, and an important change was the application of the concept of Design Extension Condition (DEC). Part of the BDBA, which was originally part of the over-design basis accident, is included in the scope of design, and corresponding design requirements and guidelines are proposed to improve the plant's ability to deal with accidents that are more severe than the design basis accident, and to realize the safety goal of practical elimination. Therefore a list of design extension conditions needs to be identified first in order to facilitate the development of measures for these accident scenarios. In this paper, a sequence identification approach for design extension conditions with core melting is researched and determined by investigating the vulnerability analysis methodology and the design extension conditions analysis methodology. Through the combination of deterministic, engineering judgment and probabilistic methods, the design extension conditions with core melting are identified and practiced in the design of HPR1000.