Based on the water quality investigation in East Lake Taihu from April 2002 to April 2003, a preliminary analysis was carried out on the water quality, its changes and the spatial difference in given districts. The mean value of the water quality parameters ranged as follows: TN, 0.57-3.91 mg/L with a mean value of 1.61 mg/L; NH₄⁺-N, 0.01-0.42 mg/L (0.14 mg/L on the average); N0₃^--N, 0.044-0.549 mg/L, (0.16 mg/L on the average); and the mean TP was 0. 103 mg/L. Compared with the results of earlier investigations, TN, TP and NH₄⁺-N had increased, COD_Mnhad slightly decreased . At present, the water quality was bad, but different sites exhibited different conditions. The water quality in the center of lake was the best. As a whole, East Lake Taihu was meso-eutrophic. Some part of the lake was eutrophic. To improve the water quality of East Lake Taihu, it is important to shut off the external pollution sources, ensure the function of East Lake Taihu, control the scale of pen-culture and recover and protect the aquatic vegetation.