Subsynchronous Oscillation problems caused by HVDC system are studied and analyzed intensively in this paper based on eigenvalue analysis method. By establishing the small signal linearized model of a typical HVDC system, subsynchronous oscillation characteristics of the system with or without SSDC are obtained. Further more, the influence of SSDC parameters to the system subsynchronous oscillation characteristic can be illustrated clearly. This is significant for SSDC design in order to achieve a satisfied restraining effect. Comparing with more accurate electromagnetic transient simulation results, the consistency of the two methods is verified and it can be demonstrated that eigenvalue analysis method is adequate for studying subsynchronous oscillations.