To deal with the subsynchronous oscillation in the ACDC interconnecting system, the complex torque coefficient analysis based on the damping characteristic is utilized to investigate the mechanism of the Subsynchronous Oscillation in this paper. Firstly, a small signal model is built to derive the relationship between the variation of angular speed and the electromagnetic torque. Then, the impacts of the PI control parameters, VSC control strategy and DC line length on the oscillations are investigated, and the testing signal methodology is utilized to obtain the system damping characteristics. Finally, the analytical results are verified by the simulations performed in PSCAD/EMTDC. It can be concluded that SSOs are much easier to be triggered when the high proportional coefficient and low time constant are employed. The AC voltage control strategy at the sending end of VSC has the better SSO damping performance than the reactive power control strategy. Moreover, the damping performance could be improved by long DC transmission lines, which mitigates the DC voltage fluctuation in the HVDC system.