It is essential to consider ecosystem services (ESs) trade-off/synergy relationships and their change thresholds when evaluating the benefits of regional ecological restoration projects (ERPs) and informing conservation management decisions. Prior research has mainly focused on how ERPs altered ESs and their trade-offs/synergies, with little attention paid to the ecological restoration threshold associated with ESs' relationships. In this study, we used ecological models to estimate vegetation net primary productivity (NPP), soil conservation (SC), and water yield (WY) in Shaanxi Province, a representative research region that has implemented the Grain for Green Project (GGP) since the late 1990s. We also constructed a new ESs benefit index that integrated ESs and their trade-off/synergy relationships to evaluate the ESs benefits from the GGP. Additionally, we applied the methods of the geographical detector and constraint line to identify the dominant driving forces and their nonlinear effects on the ESs benefits. The results showed that (1) the NPP, SC, and WY increased by 39%, 4%, and 42%, respectively, between 2000 and 2020. There was an obviously spatial heterogeneity in the ESs changes. The rise of NPP and SC was mostly concentrated in the center of Northern Shaanxi, the west of Guanzhong, and the west of Shannan, and the increase of WY was predominantly concentrated in the western part of Guanzhong and the central and western parts of Shannan. (2) All pairs of ESs in Shaanxi Province exhibited the strongly positive correlations, indicating a high level of ESs synergies. Spatially, the Guanzhong-Shannan border area was the primary location for the increase in the synergy degree between NPP and SC, as well as NPP and WY, and the central Northern Shaanxi was the main region where the synergy degree between NPP and WY, as well as SC and WY, decreased. (3) The ESs benefits in Shaanxi Province grew dramatically between 2000 and 2020; the significant increase in the ESs benefits was mainly concentrated in the center of Northern Shaanxi and the west of Shannan. (4) Precipitation and NDVI were the dominant factors influencing ESs benefits; both drivers had nonlinear constraint effects on ESs benefits. Specifically, the ESs benefits increased with precipitation and NDVI at first, but when the critical thresholds of NDVI and precipitation exceeded 0.6-0.7 and 1100 mm, respectively, the ESs benefits stopped increasing. Our study established a novel methodological framework for assessing the ecological benefits of ERPs. The findings provided valuable insights into ecological conservation and management in Shaanxi Province.