This letter considers the closed-loop stability issue of droop-based voltage control for wind power plants (WPPs) under varying grid strength. We first propose a data-driven online system equivalent/identification method (DOnSE) for WPPs using the weighted least-squares estimation (WLSE). Then, upon the analysis of link between stability boundary of droop gain and external grid strength, a self-adaptive gain-scheduling droop voltage control scheme for WPPs based on the DOnSE is developed to enhance the closed-loop stability and reactive power utilization of wind turbines (WTs).