Since the 1960s the water pollution is pricked up in Taihu Lake.The water quality is decreased about one degree per 10 yeais, 70% of the rivers and lakes are polluted, and 80% of river water quality can not fit the need of national quality standard of Class Eof surface water.At present, the whole lake' water is eutrophication and part of the lake is heavy eutrophication. The "Zero" actions have good effect on lightening the water pollution in Taihu Lake and keep within limits the water quality decreasing in some degree.The COD concentration of input rivers was decreased 26.7% averagely from the year of 1997 to 1998.Among the input rivers, the river of COD concentration maximum decrease was at Wutangqiao-39.6%; the river of COD concentration minimum decrease was at Hexi-1.8%.Otherwise comparison with COD concentrations of input rivers in July, 1998 and 1997,the average total decrease was-14.8%.In 1998 the decrease was-39.8% in September-31.9% in November,and-20.5% in December respectively. The COD quantity from watershed to Taihu Lake was decreased by 19.1%.The average COD concentration in Taihu Lake was decreased by 21.2% After the "Zero" actions,in the year of 1999 and 2000,the water qualities in Taihu Lake are deteriorated again.Thus to control the water quality radically other countermeasures are needed except "Zero" actions.