Abstract: When the AC grid connected with the multi terminal flexible DC transmission system fails, the voltage at the common coupling point of the system will drop and the overcurrent of AC side will rise sharply, resulting in the imbalance of the input and output power of the DC power grid. The unbalanced power causes the system DC voltage to deviate, which may trigger the overvoltage protection of the converter station, making stations out of the grid. When the AC fault occurs, if the converter station can maintain the connection with the DC grid and keep transmitting active power, that is, it has AC fault ride-through capability, the stability of the power system will be greatly improved. Therefore, AC fault ride-through control strategy of VSC-MTDC system is proposed. After an AC fault occurs, reactive power will be added to maintain the stability of voltage at the common coupling point of the system. Then, the controlling system will automatically adjust the current limit value according to the given value of reactive power to suppress the overcurrent of AC system. Meanwhile, an auxiliary circuit is designed to dynamically change the unbalanced power consumed by the unloading resistance by controlling the DC capacitor voltage in the auxiliary circuit, so as to maintain the stability of DC voltage and improve the AC fault ride-through capability.