Abstract: Based on the typical small reactors, the blackout accident sequence in small reactors under full power operation is calculated by the MELCOR, and the effect of mitigation measures is analyzed and compared in the paper. The results show that when losing the whole electrical source, the reactor loses the thermal trap and the high-pressure CMAs happens, and the integrity of small reactor is destroyed. If the emergency electrical source can be supplied in time, safety injection can be function, the recycle period which used sea water can cool the reactor and mitigate the accident process effectively.