Abstract: Torsional vibration of thermal power unit may be induced by the control system of wind farm and line commutated converter based high voltage direct current (LCC-HVDC) for wind-thermal-bundled power transmission through HVDC system. First, by using complex torque coefficient and eigenvalue analysis methods, it is identified that LCC-HVDC constant current and wind turbine DC voltage outer loop controllers are the key control loops affecting the shaft-control interactive mode. Then, based on the system transfer function, multiple key control paths which reflect the influence of LCC-HVDC constant current and wind turbine DC voltage outer loop controllers on shaft-control interactive mode are extracted, including HVDC constant current control path, wind turbine DC voltage control path and interactive control path. Then, the damping coefficients of shaft-control interactive mode provided by different paths are calculated quantitatively. The supplementary damping control is designed to improve path damping coefficients via phase compensation on key control paths, thereby suppressing torsional vibration. Finally, the PSCAD/EMTDC simulation results verify the correctness of theoretical analysis and the effectiveness of the oscillation suppression strategy.