The greenbelt is transforming from an ecological isolation and growth boundary to a composite green space that combines ecological security, public services, and green value transformation in high-density cities. Taking the greenbelt in Shanghai and its 2-kilometer surrounding area as the research area, a social-ecological-economic performance evaluation index system was constructed. Coupled with the coupling coordination model and spatial statistical methods, the performance coordination pattern and influencing mechanism of the greenbelt were evaluated. The results showed that the mean coupling coordination degree of the study area was 0.619, with the overall coordination mainly being moderate and basic, while highly coordinated areas are relatively few. The coupling coordination degree showed significant spatial clustering characteristics, and the low-coordination coldspots were highly correlated with areas of low economic performance that lagged significantly, and the area dominated by economic performance obstacles accounted for 85.59%. This indicates that insufficient green space value transformation is the main factor restricting high-level coordination. The analysis of the influencing mechanism indicates that the insufficient performance coordination of the greenbelt mainly stems from the poor connection of the "ecological supply-social use-value transformation" chain. The optimization of the greenbelt should be carried out under the constraint of ecological bottom lines, with a focus on enhancing the accessibility of the greenbelt, improving the connection of public services and pedestrian systems, moderately cultivating leisure services, and promoting the coordinated improvement of social-ecological-economic performance.