Abstract: Under the dual pressures of global warming and ecological degradation, establishing an “intelligent, streamlined, and efficient” green watershed assessment system has become a core issue for sustainable development. Taking the Ganjiang River basin as a case study, this paper proposes a three-dimensional, nine-level framework integrating “temporal - spatial - spatiotemporal” dimensions. It couples AHP-EWM, M-K trend and abrupt change tests, piecewise linear fitting, Moran’s I, LISA clustering, Dagum Gini, standard deviation ellipses, and triple convergence tests (σ /α /β) to comprehensively measure the Green Watershed Level (GWL) across 11 cities in Jiangxi Province from 2003 to 2022. The GTWR analysis reveals the driving patterns of six factors: human-water-energy-carbon-green-urban. Results indicate: Temporally, GWL exhibits sustained growth without abrupt changes, transitioning from slow to rapid growth since 2016. Spatially, GWL shows higher values in the south and lower in the north, with predominant “high-low” and “low-high” local clusters. Regional disparities decline in an “N”-shaped pattern, reflecting spatially unbalanced convergence. In spatiotemporal coupling, the GWL center of gravity shifted southwestward, with distribution patterns evolving from clustering to dispersion. The GWL of the 11 cities progressed from a median value in 4 cities and a low value in 7 cities in 2003 to a high-value range of 0.6–0.8 across all regions by 2020. Triple convergence of σ, α, and β confirms that the GWL of the 11 cities is moving toward spatiotemporal equilibrium. Regarding driving mechanisms, the marginal effects of six factors successively peaked and declined: population and carbon productivity shifted from positive to negative, water and forest marginal dividends rapidly diminished, the positive effect of energy expanded at a slower pace, and the urbanization effect passed its inflection point and approached zero. These findings provide historical references and precise policy guidance for the high-quality development of the Ganjiang Green Basin, while also offering methodological insights for other provinces in establishing green basins.