The southwest China is the ecologically fragile area with the strongest karst development in China and the ecologically restored area with remarkable effect of reforestation. Although some studies have confirmed the causal relationship between the implementation of ecological restoration projects and the improvement of ecosystem services, the complex relationship between multiple ecosystem services and the constraint effect of vegetation restoration on ecosystem services were rarely studied. Therefore, we accurately estimated Net Primary Productivity (NPP), water production, grain production and soil conservation services in the Southwest China, constructed the relationship index among ecosystem services. The temporal variation characteristics and driving forces were analyzed, and the constraint effect and impact threshold of the relationship between forest cover and ecosystem services were identified. The results showed that: (1) From 2000 to 2018, the mean value of regional ecosystem service synergy index did not change significantly, the tradeoff index showed a decreasing trend, and the comprehensive index increased slightly. (2) The spatial distribution of the relationship index among ecosystem services was significantly different. The increase of synergy index was mainly concentrated in the southern part of the study area. The decrease of the tradeoff index was mainly found in central Sichuan and eastern Guizhou, but the increase of the comprehensive index was mainly distributed in Yunnan, eastern Guizhou and southern Sichuan. (3) The change of ecosystem services' relationship was influenced by many factors, among which, the covering-forest had the most prominent effect, and the effect of each factor had obviously spatial unbalance. (4) There were certain constraints on the synergistic, tradeoff and comprehensive relationship between forest cover and ecosystem services. When the impact threshold exceeded 59%, the expansion of forest cover did not bring about the continuous development of ecosystem services. Therefore, it is necessary to rethink the ecological restoration scheme that only relies on expanding forest area to improve the comprehensive capacity of ecosystem services, and build a comprehensive and strategy for the ecological restoration.