Based on the novel and controllable characteristics of VSC-HVDC, an improved double close-loop decoupling control strategy is proposed to utilize the supporting capability of VSC-HVDC for frequency and voltage of weak AC systems. A power outer-loop with AC frequency and voltage additional control functions is designed to help the fast recovery of AC frequency and voltage after disturbances. Then, an adaptive droop control with power deviation feedback is adopted to improve stable operation range of VSC-HVDC. As an example, a typical weak AC system fed by a VSC-HVDC link, including a synchronous generator with an exciter, a governor and a turbine is simulated. The time-domain simulation results demonstrate that the proposed control strategy can seamlessly handle transitions between different control modes and effectively enlarge the stability operation extent of VSC-HVDC systems.