[Objective] The spatiotemporal changes and trade-off/synergistic relationships of ecosystem services in an ecological county were analyzed in order to provide a theoretical reference for promoting the construction of national ecological civilization and to realize the coordinated development of ecological protection and social economy. [Methods] This study was conducted at Wuning County of Jiangxi Province. Field-measured data were used with the InVEST model to evaluate four typical ecosystem services from 2000 to 2020: water conservation, soil conservation, carbon storage, and food supply. On this basis, the trade-off and synergistic relationships of ecosystem services and their spatiotemporal changes were determined. [Results] ① From 2000 to 2020, the average annual water conservation per grid unit at Wuning County increased from 173.14 mm to 283.92 mm, and the average annual food supply per grid unit increased from 3.52 kJ to 5.34 kJ. The soil conservation function showed a fluctuating trend of decreasing, then increasing, and then decreasing. Over the past 20 years, the soil conservation function decreased by 15.71%, while the carbon storage function showed a weak growth trend. ② The forest land at Southern and Northern Wuning County was a high value area of water conservation, soil conservation, and carbon storage. The water areas, construction land, and unused land were the low value areas for water conservation, soil conservation, and carbon storage. Forest land, and farmland were the key land use types for the development of ecosystem service functions. ③ Water conservation, soil conservation, and carbon storage in Wuning County were synergistically related, but the relationships between food supply and the other three ecosystem services were trade-off relationships. The synergies of “high-high” and “low-low” agglomeration were mainly located in high-altitude forest land and water areas, while the trade-off relationships of “high-low” and “low-high” agglomeration were mainly located in farmland. [Conclusion] During 2000—2020, the overall ecosystem service function of Wuning County has shown an upward trend, and the conversion of ecological regulation service functions into resource values should be emphasized in the future.