Abstract: As an important barrier to nuclear safety, nuclear island pit bosses need to maintain their integrity after a severe accident. Research by scholars at home and abroad on the safety of the nuclear island pit boss under the combined action of earthquakes and loss of coolant accidents (LOCA) in severe accident conditions remains relatively limited. Therefore, refined finite element models of different types of nuclear island pit boss components were established using ABAQUS. By simulating the most unfavorable working conditions for the combined action of earthquakes and LOCA, the stress distribution characteristics, deformation laws, and failure modes of various boss components under combined loads were systematically explored. The results show that different types of bosses can maintain structural integrity under the most unfavorable working conditions, and the arrangement of the steel bars can effectively improve their bearing and deformation capacities. The relevant conclusions can provide a reliable theoretical basis and technical support for the optimal design, safety verification, and emergency disposal of nuclear island pit bosses and further enhance the safety resilience of nuclear power plants under extreme combined working conditions.