Abstract: Using MELCOR code, the combination of LOCA and blackout accident of ship reactor was modeled and calculated, and the accident process and source term release were researched. The results show that the accident leads to lower head of pressure vessel and bilge creep-rupture finally without emergency power. The release fraction of inert gases and iodine are above 80%, the main form of iodine is CsI with great deposit and less airborne fraction. The accident process is decided by the equivalent diameter of break size. The production of H₂ is decided by core temperature and water remaining in the core, but has nothing to do with equivalent diameter of break size. The probability of H₂ detonation is unlikely to occur. The results can provide technical support for emergency maintenance and emergency decision-making.