The establishment of the Three-river-source National Park is beneficial because the strictest ecological protection extended to this area helps to strengthen the sustainable protection of the "Chinese water tower" and to enhance the national ecological security barrier. In order to understand the ecological background of the Three-river-source National Park, the spatial distribution of the ecosystem, the temporal and spatial variation of ecological functions from 2000 to 2015, and the importance hierarchy of the ecosystem functions were analyzed by using remote sensing data, a geographical information system, and model simulations. The results showed that:(1) Grassland, desert, water, and wetland ecosystems are the main ecosystem types in the Three-river-source National Park, and they accounted for 99.8% of the total area. (2) Areas with extremely important water regulation, soil conservation, wind prevention/sand fixation functions accounted for approximately 15.3%, 13.7%, and 22.4% of the total area in the Three-river-source National Park, respectively. (3) In the eastern, central, and western parts of the Three-river-source National Park, the core ecosystem functions were the water conservation, soil conservation, and wind prevention/sand fixation functions, respectively. (4) From 2000 to 2015, the water conservation volume decreased overall, while the increased area accounted for 84.5% of the total area, and the area of extremely important site decreased. The overall improvement of the soil conservation function was significant both in volume with a change of 9.87×10⁶ t/a and in the improved area, which was more than 95%. However, the wind prevention/sand fixation function showed a declining trend, because of the decline in wind speed and vegetation coverage, with a change of -3.56×10⁶ t/a. (5) A warm and wet climate and the implementation of the ecological protection project were the main reasons for the improvement of ecosystem functions in the Three-river-source National Park. However, grassland degradation was not fundamentally reversed, and vegetation coverage still declined regionally. Thus, the restoration of degraded grassland must be prioritized and the integrated capacity of ecosystem functions remains to be improved. For the strict protection and sustainable use of the Three-river-source National Park and its natural resources, overall planning and scientific layout should be scrutinized, and classifications and subarea protection should be implemented based on the natural law of the ecosystem.