The marine nuclear power plant is a floating nuclear power plant which supply power for the offshore oil drill platform. It is designed based on the standard and experience of the traditional onshore nuclear power plant. The reactor of the marine nuclear power plant is a 100 MWt PWR type small modular reactor with 2 loops. Each of the loops contains a main pump, a main check valve, and a steam generator. The pressurizer is set on one of the loops. The engineering safety features in the plant are somewhat different from those in the traditional plants. The special residual heat removal system, the passive residual heat removal, the square steel containment and containment suppression system are designed in the plant. The prevention and mitigation measures for severe accidents are set up on a reasonable and feasible basis to actually eliminate the large release of radioactive products. A MELCOR model and corresponding input deck were developed for the reactor coolant system, the secondary system, the containment system, the engineering safety features. Based on the safety analysis experience in the traditional nuclear power plant, the SBLOCA in the cold leg with the break diameter of 2.5 cm is chose as the initial event of the severe accident in the work. The sequence and important parameters in the accident are analyzed. According to the simulation results, the core exposed and heat up with the coolant release, and finally melt since the emergency core cooling system failure. However, the reactor pressure vessel (RPV) maintained integrity with the mitigation measure, i.e., the external vessel core cooling. The containment also maintained integrity, which prevented the large release of radioactive products to the other cabins and the environment. The work is useful in gaining an insight into the detailed process involved. One of the final goals of this work would be to identify appropriate accident management strategies and countermeasures for the SBLOCA induced severe accidents during the design process of the marine nuclear power plant.