Abstract: The lower support plate was used as the reactor internal component to carry the weight of the whole core. The failure process of the lower support plate under the serious accident of the nuclear power plant played an important role in the development of the accident process and the effectiveness of mitigation measures. According to the structural characteristics of the lower support plate, combined with the melting process of the core under serious accidents, three failure modes of the lower support plate were studied and determined, including mechanical failure, mechanical melting mixed failure and melting failure, and four important factors affecting the failure mode, such as temperature, melting material, water environment and melting position, were studied and analyzed, so as to provide the basis for the behavior and phenomenon research and nuclear safety assessment of core melt under serious accident.