To examine the relationship between water quality in the Niyang River and its response to land use at varying spatial scales, we analyzed water quality data from seven monitoring stations along the main stem of the Niyang River. This study evaluated the current status of water quality and its correlation with land use structures within buffer zones at different scales, identifying the optimal buffer scale for assessing the impact of land use on water quality. The results indicate that the water quality of the Niyang River falls under Class III standards, with chemical oxygen demand (COD) being a key indicator of potential pollution risk. There is a significant correlation between specific water quality parameters and land use patterns across different buffer scales. Notably, the 3-kilometer buffer zone exhibits the highest explanatory power for spatial variability in water quality, making it the optimal scale for managing water quality in the Niyang River. Forest land within this buffer zone significantly influences COD and total nitrogen (TN) concentrations. This study underscores the importance of optimizing land use structure within the 3-kilometer buffer zone, particularly through strategic allocation of forested areas, to enhance pollutant retention and improve the overall water ecological environment of the Niyang River.