China has experienced extensive afforestation over the recent 20 years. The observed terrestrial vegetation greening in China, characterized by increasing leaf area index (LAI), is mainly associated with afforestation illustrated by increasing tree cover fraction. The increasing tree cover fraction not only contributed directly to greening but also impacted the vegetation growth conditions. Quantifying the combined direct and indirect effect of tree cover fraction change on vegetation greening is challenging and not yet conducted. Here we propose an index, namely the sensitivity of greening to afforestation (SGA= ∂LAI/∂TCF) to evaluate this effect. Comparing satellite-based land cover observations of 2003–2012 with 2010–2019, SGA in China increased by 1.03, 0.49, 3.3, and 2.46 (10−2 m2 m−2 %−1) for evergreen needleleaf, deciduous broadleaf, evergreen broadleaf, and mixed forests, respectively. This indicates greening (increasing LAI) becomes more sensitive to afforestation (increasing tree cover fraction), particularly in evergreen broadleaf forests. To explore possible eco-hydrothermal factors contributing to the observed SGA increase, a diagnostic framework synergizing atmospheric supply (precipitation and potential evaporation) and biological use (vegetation transpiration) of water and energy was constructed. The results suggest that an eased water-energy constraint mostly facilitates the observed SGA increase.