The Floating nuclear power plant (FNPP) is a kind of marine nuclear power plant which takes barge as carrier and is fixed in specific sea area by mooring device. A 100 MWt PWR type small modular reactor (SMR) with 2 loops is used in the FNPP. It is designed based on the standard and experience of the traditional onshore nuclear power plant. 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. Three severe accident mitigation measures, i.e., the pressurizer (PZR) relief extension, the external vessel reactor cooling (EVRC) and the Passive autocatalystic recombiners (PAR) are set up to actually eliminate the large release of radioactive products. The severe accident induced by station black out (SBO) for the SMR in FNPP is analyzed with MELCOR code in the present work. The MELCOR model were developed for the main systems in the SMR, including the primary and secondary system, the engineering safety features, the containment system, and the severe accident mitigation measures. The severe accident sequence as well as the key phenomena during the accident are analyzed. The simulation results indicated that the core exposed and heat up for the coolant leakage via the pressurizer safety valve, and finally melt with the high pressure and lower pressure injection 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 severe accident process involved, and could be used to identify appropriate severe accident management strategies and countermeasures for the FNPP.