According to the new version of the HAF-102 regulation in China, a set of design extension conditions (DEC) must be derived based on engineering judgment, deterministic assessment, and probabilistic assessment, in order to further improve the ability of nuclear power plants to mitigate more severe conditions than design basis accidents. For the design extension condition with core melting (DEC-B), engineering judgment and deterministic assessment are more commonly used for sequence selection in China, while PSA only provides limited support. In fact, PSA, as an important tool, can identify the accident sequence spectrum that causes core melting, thereby providing more support for the determination of DEC-B sequences. This article will propose a method for determining the preliminary DEC-B list based on the PSA model, and take a power plant as an example to discuss how to use this method to determine the preliminary DEC-B list, as well as the process of determining the final DEC-B list based on engineering judgment and deterministic assessment.