Abstract: In the wind-thermal bundling system, the integration of wind turbines makes the sub-synchronous oscillation of thermal power units more complicated. In this paper, based on the first standard model of IEEE, the wind-thermal bundling system model is built, and the relationship between the PMSG output and the sub-synchronous damping of the thermal power unit is derived. The mechanism of its influence on the sub-synchronous oscillation of the thermal units is analyzed, and verified by the complex torque coefficient method and the time-domain simulation analysis on the PSCAD platform. Based on this mechanism, a suppression method of additional damping control at the outer control ring of the PMSG grid-side converter is proposed, and the optimization designing method for the key parameters configuration is analyzed. The time-domain simulation results show that the additional damping control can effectively suppress the sub-synchronous oscillation of the thermal power unit.