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Payment for Ecosystem Services (PES) is now widely accepted as a policy tool for advancing both ecological and social progress. The Chinese government's Grain to Green Program (GTGP) is the world's largest PES program. It was initiated nearly 20 years ago to reverse previous environmental degradation. However, it is difficult to achieve both conservation and economic "win-win" gains, and care is needed in monitoring environmental and socioeconomic outcomes. Here we choose the Loess Plateau (LP), the area where GTGP was implemented most intensively, to study the social-ecological changes after GTGP and determine whether these changes are related to GTGP. The results show that LP has achieved "win-win" gains of restoring environment and promoting socioeconomic development: normalized difference vegetation index (NDVI) time series show a significant greening trend, and soil retention and carbon sequestration services have improved from 2000 to 2015. Simultaneously, grain output from the LP has increased by 56.7%. We also analyzed the effects of afforestation on changes of socio-ecological factors, and find that the changes of NDVI, soil erosion, and carbon sequestration mainly reflect the degree of afforestation rather than climate change, while the correlations between changes of socioeconomic factors and degree of afforestation are insignificant. We conclude that, despite some adverse outcomes such as water yield reduction, GTGP in LP has achieved considerable overall success in environmental factors. However, comprehensive and quantitative evaluation of the social impacts of GTGP is needed, as the impacts are clouded by multiple socioeconomic factors. (C) 2019 Elsevier B.V. All rights reserved.