The distributed power flow controller (DPFC) has good power flow control and convenient installation and operation. This has been demonstrated and applied in many projects in China and abroad. In order to analyze the damping mechanism of a DPFC, this paper constructs the system equation with a DPFC based on a simple power system. The linear state equation of a simple power system with a DPFC is derived by taking the amplitude and phase angle of the DPFC output voltage. These represent the modulation depth of the DPFC sub-unit as control variables. The expressions of amplitude and phase angle of the DPFC output voltage for the control effect of the electric angular velocity increment of the system generator are constructed, as are the transient electromotive force of the AC shaft and the forced no-load electromotive force. The criterion that the DPFC has the ability to change the damping speed is proposed. It is concluded that when the total output voltage of the DPFC increases in the direction of capacitive voltage, the system damping will also increase. Based on the output voltage control of the DPFC, the additional damping controller of the DPFC is designed. The simulation results show that the proposed method can effectively pull the divergent oscillation back to the new stable point in a short time.