The flexible DC interconnection technology can effectively improve the power quality of the power grid,and enhance the reliability of power supply,thus improving the level of new energy consumption. In a flexible DC interconnection system with master-slave control,sudden load increase and short-term fluctuation in new energy output are likely to cause the actual power of the master station to exceed its capacity. It is difficult to timely suppress power fluctuation through traditional power limiting or scheduling methods,which leads to abnormal DC side voltage and threatens the stable operation of the power system. Therefore,a master-slave control strategy that takes into account the capacity margin of the master station is proposed. The strategy optimizes the operation mechanism of the master station and incorporates the capacity margin factor into the operation mode of the master station to adjust the voltage of the master station. The loop structure and variable definition of the loop are added. A closed suppression loop based on overcapacity power is added,which ensures the continuous regulation ability of the main station to the DC bus voltage. Since the master station needs the coordinated operation among the slave stations when balancing the system power,a power support mechanism among the slave stations is introduced based on the traditional operation mode and original scheduling of the slave stations,and a system under the constraints of system power balance and other constraints is established. The multi-objective optimization model with the smallest system economic loss and the smallest source-storage equipment loss is established,and the non-dominated sorting genetic algorithm-Ⅱ (NSGA-Ⅱ) is used to solve the optimization model quickly. Finally,it is verified by simulation that the proposed strategy can effectively prevent the overcapacity of the master station and ensure the stable and efficient operation of the flexible DC interconnection system.