Aiming at the problem that when failures occur in AC side of MMC -MTDC system, the voltage at the point of common coupling (PCC) between DC system and power grid also drops, an AC fault ride -through technology is proposed to maintain voltage stability of PCC. According to the voltage sag degree of PCC, the corresponding reactive power is increased to maintain the voltage stability of PCC and ensure the active power transmission of the system. When the voltage sag of PCC is large, the additional reactive power will cause the overcurrent of AC system. So it is proposed to reduce the reference value of active power at the fault end, thus reducing the current amplitude in AC side and avoiding the generation of overcurrent. At the same time, in view of the phenomenon that the reduction of active power will further increase the unbalanced power of the system, resulting in large fluctuations in DC voltage, the unbalanced power of the system is absorbed by the constant DC voltage converter station based on the change of DC voltage, thereby to achieve the purpose of maintaining DC voltage stability of multi -terminal flexible DC transmission system.