With the scheduled back to back VSC-HVDC based unsynchronized interconnection between Chongqing and Hubei power grid, the dynamic of such hybrid AC-DC power system consist of large capacity hydro-generator units and both LCC and VSC based HVDC is of serious concern. An equivalent system and the detailed time domain model are established including Three Gorge Power Plant No. 1 located at left bank, the back to back VSC-HVDC and the Long-Zheng LCC-HVDC. Transient simulation is undertaken which demonstrates post fault power oscillation of the hydro-generator units with poor damping. It is proposed that damping controllers can be added to either the activeor reactive control loop of VSC-HVDC to improve the damping of the power oscillation. These two ways of damping control design are studied and validated. Results show that combination of both design, i.e., damping controls based on active control and reactive control, could fast decrease power oscillation as well as stabilize voltage variation.