Abstract: Because of the lake's self-purification capability, the lake can withstand a certain degree of pollution. But pollutants caused accumulation of the pollutants, which Lead to excessive growth in water algae. The algae can cause the decline of DO content. Taihu has many functions, such as the flood storage, water supply, irrigation, navigation, travel etc functionality. And it is the important water supply water source to many areas. So it is important that protected and improved the taihu lake. This paper take the taihu lake basin as an example, Selected three indexes, dissolved oxygen, Potassium permanganate index and ammonia nitrogen to analysis the water quality variation characteristics of taihu lake basin in recent years. Water quality has been classificed by FCM algorithm. The result has been displayed that the water status remained relatively grim, still need to strengthen the management to improve water quality.