To address the issues of high operational costs and unclear water source composition in the South-to-North Water Diversion Eastern Route Project, an optimal dispatching model was developed. Comparative analysis was conducted on the water supply volume and pumping water volume from the Yangtze River under the current and optimal dispatching scenarios. Additionally, a source water partitioning method was employed to analyze the water source composition across different hydrological years in various sections under the optimal dispatching scenario. The results show that compared with the current dispatching scenario, the optimized dispatching scenario reduces pumping water volume from the Yangtze River by 5.0% in extremely wet years by increasing the utilization of reservoir storage from source lakes. In extremely dry years, it reduces pumping water volume from the Yangtze River by 3.6% while supplying an additional 6.717 million m³ of water supply volume to the receiving regions. Under the optimized dispatching scenario, Yangtze River water remains the primary supply source across all sections. During the extremely dry years, the proportion of Yangtze River water in each section increases, with the Luoma Lake-Dongping Lake section reducing the proportion of Hongze Lake water and Luoma Lake water while incorporating the Lower Lake as an additional water source. During the extremely wet years, the proportion of Yangtze River water has been reduced in each section, while the proportion of Hongze Lake water and Luoma Lake water has been increased.