Abstract: In this paper,through the mathematical model of the transient process of the flexible DC system,the unscented Kalman filter is used to track the output current of the modular multilevel converter（MMC）AC side under fault conditions;and the fault current is divided into the steady-state maximum current and the uncontrollable current. The uncontrollable current is eliminated on the shaft,so as to achieve the purpose of suppressing the fault current. Finally,a simulation model is built through the software platform to verify the effectiveness of the proposed method.