The nuclear safety is the basic demand for the development of nuclear energy. The severe accident which is the design extended conditions that can lead to core melt in the nuclear power plant, is of significant influence to the NPP design. The risk of severe accident in marine nuclear power plant (MNPP) is higher than that in the onshore NPP for the limitation of marine condition and the limited layout space. Thus, the severe accident mitigation measures are much more important in the design. The mature severe accident research method for onshore NPP is referenced for the MNPP research in the work. The reactor of the marine nuclear power plant is a 100 MWt PWR with 2 loops. 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 mitigation measures for severe accidents are set up on a reasonable and feasible basis to actually eliminate the large release of radioactive products. The pressurizer (PZR) relief extension is the measure to decrease the primary system pressure, avoiding the high pressure core melt. The external vessel reactor cooling (EVRC) is the measure to achieve the molten material in vessel retention, maintaining the reactor vessel integrity. The Passive autocatalystic recombiners (PAR) is the measure to decrease the hydrogen concentration in the containment, avoiding the hydrogen explosion. The MELCOR model and corresponding input deck were developed for the reactor coolant system, the secondary system, the containment system, and the mitigation measures. The typical severe accidents of the MNPP were chosen to evaluate the effect of three mitigation measures. The severe accident sequences and important parameters for the accident with and without the mitigation measure are comparatively analyzed. The influence of different mitigation measure to the key parameters and accident progression are achieved. The results showed that all the three measures could take the scheduled function, and are significant in mitigating the accident. The results can be used to support the design of severe accident mitigation measures in the new MNPP, which is meaningful to the development of the MNPPs.