Abstract: In the process of regional development, land use/cover change exhibits significant spatiotemporal heterogeneity, resulting in phase-dependent variations in ecological environmental quality. However, existing studies often face challenges such as inaccurate identification of turning points and insufficient comparative analyses, complicating the systematic understanding of the transition mechanism of land use and ecological environmental quality. Based on the case of the Yangtze River Delta with full-length land use data from year 2000 to 2020, this study quantitatively analyze the spatiotemporal evolution of land use/cover and its ecological implications by employing integrated methods such as piecewise linear regression, land use transition analysis, and kriging interpolation methods as well as the ecological environmental quality indices and the ecological contribution rate of land use/cover change. The main findings include: ① Between 2000 and 2020, a distinct phase in land use/cover evolution and ecological occurs, with a turning point identified in 2009. ② Prior to 2009, forest and water areas slightly increased while arable land decreased; After 2009, forest and water areas began to decline, with a markable reduction in the rate of arable land loss. ③ Before 2009, ecological quality improved but exhibited minor degradation thereafter. ④ The primary drivers of land use changes transitioned from "returning farmland to forest and wetlands” before 2009 to "hillside cultivation and subsurface irrigation" afterwards, influencing the shift in ecological quality. This study provides methodological insights into the interplay between land use and ecological quality, and serves as a scientific basis for land resource optimization and ecological protection in this region.