The flexibility of flexible DC control has been widely studied to suppress subsynchronous oscillation, but there is little explanation of the mechanism for the different suppressive effects of active and reactive control loops. Aiming at the subsynchronous oscillation problem of thermal power units in the vicinity of flexible DC, a suppression strategy for subsynchronous oscillation of flexible DC based on additional damping control of the power outer loop is proposed. By analyzing the negative damping frequency band of the electrical damping curve and combining the control characteristics of the flexible DC converter, the optimal position for additional control is selected, and the electrical damping of the thermal power unit is improved through additional damping control based on the power outer loop. Then, a transfer function of the complex torque coefficient is established, and it is analyzed that the damping effect of the active and reactive outer loop with additional damping control is related to the parameters of the power outer loop, mainly related to the transmitted active and reactive power, revealing the mechanism of the difference in the impact of additional damping control on the electrical damping of the d-axis and q-axis of the power outer loop. Finally, the correctness of the analysis is verified through simulation of PSCAD time-domain model.