Since the Eastern Route of the South-to-North Water Diversion Project (SNWD-ERP) officially began operation, it was found that the concentrations of sulfate rapidly increased after water transfer from Jiangsu Province to Shandong Province, especially in Nansi Lake (NSL). The direct and original sources of sulfate in NSL and its surrounding inflowing rivers were investigated by analyzing the hydrochemical properties of the water bodies and sulfur and oxygen isotope compositions of dissolved sulfate during nondiversion and diversion water periods. During the diversion water period, the largest direct source of high SO42- concentrations in NSL was evaporite dissolution, accounting for 52.18% of the total contribution. In contrast, during the nondiversion water period, the maximum contribution of SO42- came from inflowing river inputs. To ensure the safety of the water supply of the SNWD-ERP, some measures must be taken to decrease the sulfate inputs from inflowing rivers, such as formulating strict discharge standards to reduce the direct discharge of sewage into the inflowing rivers around NSL.