Abstract: In recent years, inspired by the idea of power flow control devices developed in traditional AC transmission system, the concept of DC power flow controller was proposed and various studies on its topology implementation scheme, steady-state operation control strategy, and system operation characteristics when participating in power flow regulation have been carried out. For the control and operation of the multi-port DC power flow controller (M-DCPFC), a new M-DCPFC control method is proposed in this paper, which further expands the functionality of the M-DCPFC. On the basis of ensuring system power flow regulation requirements, this method is able to introduce virtual damping on the transmission line to suppress the sub-synchronous oscillating current in the DC network, which effectively limits the fault impact region and improves the stability of the overall transmission system. The damping control mechanism of M-DCPFC under sub-synchronous oscillation is analyzed in detail, and the implementation of the control strategy is discussed. A five-terminal flexible HVDC transmission system is developed as the test platform in the real-time model-in-the-loop simulation. The proposed damping control is validated under different cases and shows its effectiveness.