In order to explore the impact of water diversion on the improvement of water quality in the western part of the upper Taihu Lake，simultaneous monitoring of water quantity and quality during the diversion period was carried out to analyze the characteristics of the regional water flow and the water allocation，as well as the changes in the water quality of the regional water network under the water flow during the whole period.Results show that the water diversion has significant effect on improving the water quality of the water network in the upper reaches of Taihu Lake，with about 75% of the monitored sections having improved water quality；the degree of improvement in water quality in the northern and western parts of the region is higher than that in the southern and eastern parts of the region，respectively；the degree of improvement in the water quality of the water network in the middle and late phases of the diversion is greater than that during the initial period.