Abstract: Extreme middle break loss of coolant accident (MBLOCA) occurring at cold leg was analyzed with the RELAP5/MOD3.2 code for a marine reactor, and different accident processes were simulated separately under the conditions of cold leg safety injection and hot leg safety injection. Based on the concept of critical pipe which was introduced in this paper, the critical power factor of fuel cladding failure was given, and then pin power distribution of the whole core was calculated subsequently. Finally, the cladding failure fraction of the reactor was got. It is pointed that, during the design basis LOCA occurring at cold leg of the main loop, hot leg safety injection can reduce the accident results and the failure fraction.