The Grain for Green Project significantly changed the land use in Yan'an City, and then improved the ecological environment of Yan’an City. The evaluation of the ecological environment change of this process is beneficial to the subsequent ecological protection and planning decisions. Based on existing research, the land use types, and the natural and social statistical data from 1990 to 2020, this study explored the spatial-temporal variation and impact factors of ecosystem service value (ESV) in Yan’an City before and after the implementation of the project of Grain for Green by using temporal information entropy model and geographical detector. The results showed that: (1) From 1990 to 2020, the total ecosystem service value of Yan’an City increased by 770 million yuan, with an increase of about 32.42%. The value of forest ecosystem services increased by 1.125 billion yuan, with an increase of 80.54%, which was the main contributor to the growth, and the value of regulatory services accounted for 50%. (2) The overall spatial distribution pattern of ecosystem service value in Yan’an city was "high in the south and low in the north". ESV increased in 25.1% of the region, distributed in the north and south of Yan'an; ESV decreased in 6.4% of the region, distributed in the suburbs of each district and county. (3) Normalized difference vegetation index (NDVI) was the main impact factor of spatial variation of ESV, and the synergistic effect of various factors jointly affected the spatial variation of ESV in Yan’an City.