To reduce the amount of discarded hydropower of Yunnan Power Grid during flood season, a feasibility study on interconnection between Yunnan Power Grid and Guizhou Power Grid is conducted by China Southern Power Grid. For the above research requirements, interaction analysis and suppression technology of sub-synchronous resonance/oscillation (SSR/SSO) in the AC-DC power grid with thermal power units, fixed series compensations and HVDC transmission system are carried out. Firstly, the frequencies of the SSR/SSO in AC system, which may be caused by the three types of above-mentioned equipment respectively, are theoretically calculated and analyzed, then the possible three types of SSR/SSO modes and their dominant sources are obtained comprehensively, which are verified through the EMTDC fault time domain simulation and comparative analysis. Further, from the perspective of HVDC additional control, an improved design method of DC sub-synchronous damping controller (SSDC) is proposed according to the characteristics and distribution of the SSR/SSO modes in the power grid. The results of the fault time domain simulation in EMTDC with a new SSDC designed using the above method prove the correctness and effectiveness of the method.