Abstract: According to the operating character of the ship reactor, the medium break loss of coolant accident (MBLOCA) combined with blackout accident was studied. The accident response sequence was established for the combined accident. The 30 mm unsegregated MBLOCA combined with blackout accident was analyzed with RELAP5/MOD3.2 code when the reactor operated at 30% rated power. The integrity of fuel elements was also judged. The results indicate that the decay heat of the reactor will not be carried out of the core because of the loss of heat trap under the combined accident, finally all the fuel elements are failed even molten. The research is helpful for the processing of the accident and the establishment of emergency regulations.