Because forest ecosystems and the services they provide cannot be replaced, the main purpose of forest maintenance and management is to protect forests, as a basic resource for human survival and development. As a case study, the present study investigated Xichuan County, which is the main source of water for the middle route of the south-to-north water diversion project, by quantitatively analyzing changes in forest resources from 2004 to 2014, using TM and GF1 image data from 2004-2009 and 2014, respectively. On the basis of previous analyses, the InVEST model was used to quantitatively evaluate the ecosystem function of the forest (water conservation, water purification, and soil conservation). The results indicated that the area of forest increased rapidly, by 10.85% from 2004 to 2009 and by 7.44% from 2009 to 2014, which resulted in a forest area of 45.65% by 2014. From 2004 to 2014, the closed forest land of the county increased as well, whereas the area of shrubland and Nursery woodland increased more slowly, and the area of sparse woodland and non-wooded land both tended to increase and then decline in succession. As the forest resources changed, significant changes were also observed in the corresponding ecosystem functions, including 10-year water purification, which increased by 29%; water conservation, which increased by 19%; and soil conservation, which increased by 6%. Meanwhile, among the individual forest types, the average water conservation function decreased annually, whereas the average water purification capacity continuously improved, and the average soil holding capacity initially increased and subsequently decreased. Forest was the main type of landscape cover in the region, as well as the main contributor to total ecosystem function, followed by shrubland and sparse forest. During the study period, areas that contributed to the county's water conservation, water purification, and soil conservation services were strongly and positively correlated with forest area, and both improvement and expansion of forested areas enhanced the ecological services provided. In addition, the spatial distribution of the forest's ecosystem function was greater in the north than in the south. Analysis of the history and current condition of forest ecosystem management in the study area indicated that the key managing forest ecosystem service lies in maintaining an appropriate distribution of forestland and other land types, and the ecological function of the forests was affected by both forest area and structure (e.g., crown density, soil thickness, soil, root depth), as well as by meteorological factors. In order to improve forest quality and productivity, future forest management strategies need to promote the expansion of pure forestland and transformation and implement changes that will improve the ability of the forest ecosystem to adapt to climate change.