In a flexible interconnected substation based on back-to-back modular multilevel converters(B2B-MMC), when the main transformer fails and exits operation, the converter needs to switch from the power transmission mode to the island control mode. The impulse voltage and impulse current generated by this process will seriously threaten the safety of equipment operation and the reliability of system power supply. For this reason, this paper proposes an improved island control mode. Based on the analysis of the operating relationship between the instantaneous power of the MMC bridge arm and the DC bus voltage, the fixed droop coefficient of the active power-DC voltage square distribution characteristic is combined with the adaptive inertial droop coefficient using virtual inertia. Among them, the adaptive inertia technology is to couple the active power of the DC side and the frequency of the AC system in the B2B-MMC system, and quickly adjust the droop coefficient to suppress the voltage shock during the switching process, reduce the peak current, and improve the stability of the system. Based on B2B-MMC, a flexible interconnected substation model is built in PSCAD/EMTDC transient simulation software, and the response characteristics of the proposed method during converter switching are analyzed. The results verify the effectiveness and practicability of the proposed method.