The cloud model was used to evaluate the changes in groundwater quality of Dongping Lake in dry seasons before and after the implementation of the South-to-North Water Diversion Eastern Route Project. Six indices, including total hardness, SO^2-₄, Cl^-, NO^-₃, NH⁺₄, and F^-, were selected as the evaluation factors. Measured data were put into the cloud model to obtain the degree of determination for each index. Furthermore, the integrated degree of determination for each sampling site was calculated based on the weight of each index. Finally, the largest integrated degree of determination was used to determine the water quality level of each sampling site. The results show that the groundwater quality of the seven sampling sites did not deteriorate after the implementation of the Eastern Route Project, and the water quality of the eastern part of the lake was worse than that of the western part of the lake.