Abstract: Based on the severe accident analysis program MELCOR as computational tool, computational model of the marine reactor was established in the paper. Both the source behavior and radioactive consequence were researched when the double-ended rupture large-break LOCA happened on the cold leg of a typical ship reactor. The release, migration and reactor cabin distribution of the noble gas and CsI were analyzed, and the plunge criterion of ventilation system was researched. The results show that in order to assure the radiation dose of the cabin adjacent reactor cabin in the dose limits, the whole ship ventilation system should be plunged within 10 min after the accident. Otherwise, the protective measure should be used before the whole body dose and thyroid dose reach the dose limits.