This study focused on the water environment improvement in the Waihai area of Dianchi Lake based on measured data before and after the Niulan River-Dianchi Lake Water Supplement Project was implemented. Water environmental indices including TP, TN, NH₃-N, COD_Mn, Chl-a, and SD were selected to assess the improvement effects in regard to the variations of index concentration, water quality classification, rate of indices that show improving values, and maximum values. The results show that TP, TN, NH₃-N, COD_Mn, Chl-a, and SD, as well as the trophic level index improved after the water supplement project was implemented, and the level of eutrophication declined significantly, indicating that the Niulan River-Dianchi Lake Water Supplement Project had a marked effect on water environment improvement.