Abstract: The first span of the conventional island plant is a unique structural design of AP/CAP series nuclear power plants, which is located between the nuclear island and the conventional island. It is equipped with the equipment cooling water pump, variable-frequency drive, battery room and other nuclear island-related equipment. In the main water supply pipeline rupture accident, the total maximum discharge amount and the maximum instantaneous discharge amount in the MFLB are related to the break size and the trigger time of the main feed water pump（MFWP）. Therefore, it is necessary to confirm the conservative working condition of the MFLB accident, so as to provide guidance strategy for the layout of depth defense equipment. In this paper, four typical break sizes are selected according to the trigger logic of four different main feed water pumps. Based on the complete model of the secondary circuit thermal system, modeling and analysis of four kinds of break sizes are carried out, and different accident sequences and dynamic discharge curves under four working conditions are obtained. According to the boundary conditions required by the nuclear island and sufficient conservative assumptions, when the main feed water pipeline breaks down, the total discharge amount of the break size Ⅱ is the largest under the accident condition and the instantaneous discharge amount of the break size Ⅳ is the largest under the accident condition.