This study investigated the impact of land use change on water quality in the Xinxian River Basin amidst rapid urbanization. While previous studies have predominantly focused on single-scale buffer analyses or specific land use types, the interactions between multi-scale riparian buffers and diverse land cover dynamics remain rarely understudied, particularly in a rapidly urbanizing county in the Yangtze River Basin. Land use type data for the Xinxian River Basin in 2000, 2010, and 2020 were acquired using GIS technology, and subsequent analysis quantified land use pattern changes over this 20-year period. Additionally, 2023 land use data for riparian buffer zones (50 m, 100 m, 200 m, 400 m, and 600 m) were obtained via GIS and subjected to Redundancy Analysis (RDA) with 2023 water quality monitoring data to evaluate the impact of land use on water quality. The results revealed significant land use conversion dynamics, particularly between natural and anthropogenic cover types. Forest cover exhibited negative correlations with riverine nutrient concentrations, while built-up areas displayed strong positive associations, especially at finer scales (50–100 m buffers). Notably, the dominant influencing factor shifted from built-up land at smaller buffer scales (50–100 m) to forest land at larger scales (400–600 m), whereas agricultural land showed no significant correlation. These findings highlight scale-dependent relationships between land use and aquatic ecosystems, emphasizing the critical role of spatial planning in mitigating urbanization impacts. The work is conducive to the sustainable development of Longgan Lake National Wetland Nature Reserve and the protection of water ecology in the middle and lower reaches of the Yangtze River.