China has undertaken extensive ecological restoration (ER) projects since the late 1970s in drylands, dominating the greening of drylands. The greening, especially ER-induced, can affect regional water availability and even cause hydrological unsustainability (i.e., lead to a negative shift in ecosystem water supply and demand balances). However, there is still limited research on accurately identifying the hydrologically unsustainable greening areas (GA) in China's drylands. Here, we developed an ecosystem water supply-demand indicator, namely, the water self-sufficiency (WSS), defined as the ratio of water availability to precipitation. Using remote sensing and multisource synthesis data sets combined with trend analysis and time series detection, we conducted a spatially explicit assessment of the hydrological sustainability risk of greening in China's drylands in the context of ER projects over the period 1987-2015. The results showed that 17.15% (6.36 x 104 km2) of the GA faced a negative shift in the WSS (indicating hydrological unsustainability), mainly in Inner Mongolia, Shanxi, and Xinjiang provinces, driven by evapotranspiration. Moreover, 29.34% (1.09 x 105 km2) of the GA, whose area is roughly double that of hydrologically unsustainable GA, exhibited a potential water shortage with a significant WSS decline (-0.014 yr-1), concentrated in Inner Mongolia, Shaanxi, and Gansu provinces. The reliability of our findings was demonstrated through previous studies at the local scale and an analysis of soil moisture changes. Our findings offer precise grid-scale identification of the hydrologically unsustainable GA, providing more specific spatial guidance for ER implementation and adaptation in China's drylands. China's drylands have been subjected to extensive ecological restoration (ER) practices, which have dominated the greening of drylands. Hydrologically unsustainable (i.e., breaking the balance between ecosystem water supply and demand) greening, especially ER-induced, may exacerbate degradation problems. We conducted a spatial assessment of the hydrological sustainability risk of greening in China's drylands over the period 1987-2015. Our findings indicated that 17.15% of the greening areas (GA), primarily located in semi-arid areas such as Inner Mongolia, Shanxi, and Xinjiang provinces, are currently facing water resource imbalances. Furthermore, 29.34% of the GA, concentrated in Inner Mongolia, Shaanxi, and Gansu provinces, may encounter similar challenges in the future. Our results facilitate the precise identification of the GA with hydrological unsustainability, providing valuable spatial guidance for ER implementation and adaptation. We conducted a spatially explicit assessment of the hydrological sustainability risk of greening in China's drylands 17.15% of the greening areas (GA) in China's drylands face the risk of hydrological unsustainability, primarily driven by evapotranspiration The significant decline in the water self-sufficiency poses a potential threat in 29.34% of the GA in China's drylands