Abstract:  Considering the critical consequences of the steam generator pipe rupture accident, in order to improve the understanding of emergency cooling system to alleviate the accident consequences, and enhance the disposal capacity in the heat transfer tube breakage accident, the safety analysis model based on MELCOR program is built in this paper. The effect of the emergency cooling system on accident consequences is calculated comparatively, and the alleviation capacity of the emergency cooling system in the accidents of for the heat transfer tubes with different sizes is compared. The simulation analysis verified the alleviation capacity of the emergency cooling system in the failure of the heat transfer tubes, and this study has great significance to improve the accident disposal ability of the operation staff, and to guarantee the safety of nuclear reactors.