Timely monitoring of urban ecological environments is essential for sustainable development. This study addresses the need to understand the complex internal mechanisms of ecological quality change, especially when indicators present contradictory trends. It evaluates the spatiotemporal evolution of ecological quality in Hangzhou, China, from 2016 to 2024, to identify the specific drivers of its improvement. The study utilized Landsat 8 data processed on the Google Earth Engine platform. Four key indicators were extracted: greenness (NDVI), wetness (WET), dryness (NDBSI), and heat (LST). These indicators were then integrated using Principal Component Analysis (PCA) to construct the comprehensive Remote Sensing Ecological Index (RSEI). The findings reveal that Hangzhou's mean RSEI increased by 8.07%, rising from 0.5249 in 2016 to 0.5710 in 2024. This overall enhancement was marked by a 9.55% expansion of the “Excellent” ecological grade area. Spatially, the city maintained a stable “southwest-high, northeast-low” ecological gradient. This ecological gain presents a significant finding. The improvement was not driven by an increase in greenness. The city-wide mean NDVI remained stable with a 0.15% decrease. Instead, the RSEI increase was caused by the mitigation of human-induced stress. This is evidenced by a sharp 17.55% decrease in dryness (NDBSI) and a 9.77% decrease in heat (LST). Hangzhou’s ecological progress was achieved by alleviating urban pressures, such as the heat island effect and impervious surface impacts, rather than by expanding vegetation. This study validates the RSEI's utility for capturing complex improvements in mature urban ecosystems.