It is essential to conduct supply and demand assessment of water ecological services across multiple spatial scales by integrating water quantity and quality for identifying water shortage issues caused by water pollution in humid regions. This study developed a comprehensive model that integrated water quantity and quality for quantifying the supply-demand relationships of water ecological services for the Taihu Lake Basin (TLB). This study also constructed three optimized scenarios (Water Efficiency Improvement, Fertilization Reduction, and Land Use Structure Optimization) to explore the spatial variations and key driving factors of water resources supply and demand on the grid, sub-basin, and county levels. We got the following conclusions:(1) The TLB was sufficient in water quantity, but it was deficient in water quality. Water efficiency improvement significantly reduced the demand for water resources in the TLB. Fertilization reduction and land use optimization greatly improved water quality. (2) The supply and demand assessment of water ecological services across the three spatial scales could help in identifying key areas for promoting sustainable water use in the TLB. (3) Urban land was primarily located in the regions with deficient water quantity. In comparison, croplands dominated the regions with deficient water quality, while these regions of deficient water quality were also comprised of large areas of urban land when assessed at the county level. (4) Land use optimization such as grain for green and riparian conservation was the most effective in improving water quality for the TLB, but it was at the huge cost of cropland losses. Therefore, coupling other optimization ways such as water efficiency improvement that would be the best measurement for solving pollution-caused water shortage issues in the TLB.