Abstract: Ecological engineering can dominate the structure and function of ecosystems. The quantity and quality of ecological land can be directly related to the pattern and dynamics of regional ecosystem services. It is very necessary to identify the impacts of ecological engineering construction on regional ecosystem services, especially in agro-pastoral transitional zones. This study aims to explore the impact of the quantity and quality of ecological land on ecosystem services under ecological projects. A case study was taken in Zhangbei County, a typical county in an agro-pastoral transitional zone. The map of land use was then classified using a random forest model, according to the Landsat remote sensing images in 2000, 2010, and 2020. The linear spectral mixture model was also used to decompose the original image for the abundance values of typical endmembers. Three key ecosystem services were included soil conservation, water conservation as well as windbreak, and sand fixation. The evolutionary characteristics were quantitatively assessed using the InVEST model and ArcGIS. The correlation analysis was then applied to reveal the impact of the quantity and quality of ecological land on the key ecosystem services. The results showed that: 1) The distribution of forestland in Zhangbei County was expanded rapidly, with a total increase of 986.10 km² over 20 years. But the areas of arable land and grassland continued to shrink, thus decreasing by 802.86 and 307.43 km², respectively. There was some spatial overlap between the expansion of forestland and the shrinkage of arable land and grassland. An overall strengthening trend was found for soil conservation, water conservation, windbreak, and sand fixation as a whole. However, there was outstanding spatial heterogeneity in the specific distribution. 2) In the quantity of ecological land, the total area of ecological land has continuously increased by 687.67 km² over the past 20 years. Among them, the increases in the area of forestland, grassland, and their total area posed a significant effect on the soil conservation and water conservation services, while only the increase in the area of forestland shared the positive impact on the windbreak and sand fixation. In the quality of ecological land, the average abundance values of endmember elements in forestland declined by 0.108 during the 20-year period, indicating an overall decreasing trend. The average abundance values of endmember elements in grassland increased by 0.099, with an overall weakly increasing trend. The quality of forestland and grassland was enhanced the soil conservation, water conservation, windbreak, and sand fixation, whereas the windbreak and sand fixation were the most sensitive to changes in the quality of forestland and grassland. The findings can offer a promising perspective to explore the impact of ecological engineering construction on ecosystem services. The region-specific decision-making insights can also be provided for ecological restoration in the agro-pastoral transitional zone. The impact of fine classification of ecological land on ecosystem services should be further examined in future research. Additionally, it is necessary to incorporate the time-series data of climatic factors into vegetation ecological process models, in order to reveal the feedback effects of ecosystem services on climatic factors. This approach can be expected to distinguish the relative contributions of climatic factors and ecological engineering in regional ecosystem services.