Contexts To quantify ecosystem service (ES) changes caused by the dynamic of green vegetation (GV) during rapid urbanization, it is necessary to fully understand the 'real' conversion pattern of GV, yet key and 'real' conversion patterns within specific periods and contributions to GV quality remain poorly understood. Objectives We use normalized difference vegetation index (NDVI) as a mediator to represent GV quality. Land cover transfer matrix (LCTM) and urban greenspace dynamic index (UGDI) were employed to fully understand GV dynamic from quantity-quality and gain-loss perspectives. The Pearl River Delta Metropolitan Region (PRDMR), one of the fastest urbanizing regions in the world, was selected as a case. Results From 1990 to 2015, built-up land, forests and grasslands has the most dynamic conversion, and also has the most significant impact on NDVI. The NDVI value of the newly-built forest (0.29) was much lower than that of the lost forest (0.5), which demonstrate the value and importance of existing natural forest ecosystem, as newly-built forest does not provide the same ESs (although newly-built forest generally has stronger carbon sequestration ability). Hence, we reconfirm and suggest that in regional ecological planning and management, in addition to creating new, higher quality GV, it is essential to protect existing natural forest ecosystems. Conclusion The study proposed new and full perspectives, including quantity-quality and gain-loss angle of view, enhance the understanding of GV dynamic and can be used in other related analyses. Results also provide important theoretical bases for regional ecological planning and natural forest ecosystem protection.