With the development of large-scale renewable energy, the wind-thermal bundled system transmitted via High Voltage Direct Current (HVDC) has become a key method to solve the problem of wind power consumption. At the same time, the problem of subsynchronous oscillation among wind turbines, HVDC, and the Synchronous Generator (SG) has become increasingly serious. Considering the dynamic interactions among Doubly-Fed Induction Generator (DFIG)-based wind turbines, HVDC and SG, a linearization model is established. The influence of the dynamic processes of wind turbines and HVDC on the electromechanical transient processes of SG is studied. Based on the method of additional excitation signal injection, when the capacity of HVDC is fixed, the influence of DFIG-based wind turbine main factors including control parameters, wind speeds, and numbers of grid connected units on the damping characteristics of the SG torsional mode is analyzed. The mechanism of the negative damping torsional mode of the SG is identified. Based on a time-domain simulation model, the effectiveness of the proposed method in revealing the influence of DFIG-based wind turbines on the shaft torsion of SG is verified. This work is supported by the National Natural Science Foundation of China Commission-Smart Grid Joint Foundation of State Grid Corporation of China (No. U1866601).