Abstract: Water diversion from the Yangtze River is one of effective measures for improving the regional water environment of Taihu Lake. According to the water diversion test data of the water conservancy projects along the Yangtze River in Suzhou city from May 9 to 18, 2017, the concentration change of main pollutants and the relationship between water diversion and water quality pollution index in Yangcheng and Dianmao districts before and after water diversion are analyzed. The results show that water diversion from the Yangtze River can speed up the water flow in the river network, significantly improve the water quality in the rivers connected to the Yangtze River and the west area of Yangcheng sub-Lake, especially, the ammonia nitrogen index can be raised 1-2 water quality classification, comprehensive pollution index also decreased. According to the analysis result, the improvement degree of water quality in various water zones of Yangcheng and Dianmao districts by water diversion are determined, which will be conducive to the water diversion scheme optimization and water environment management.