In view of the changes of Lake Taihu water exchange cycle in recent ten years, this paper collected and sorted out the hydrological survey data of Lake Taihu from 1986 to 2018, the water diversion quantity change of cities along the Yangtze River in Taihu Basin, rainfall change data of the basin, studied the changes and reasons of Lake Taihu water exchange cycle based on the changes of water inflow and outflow in Lake Taihu, and analyzed the effect of the change of water exchange cycle on water quality spatial pattern. The results show that the inflow of Lake Taihu has increased significantly, with an average annual inflow of 3.08 billion m³/a from 2007. The water exchange cycle has dropped significantly. It is about 184 days after 2007, which is 26 days lower than that before 2007. Over the past 70 years in the Taihu Basin and various water conservancy divisions, except for 2015 and 2016, there has been no significant change in rainfall. The increase in the amount of water inflow into Lake Taihu caused by changes in the underlying surface of the main lake inflow area is estimated to be about 2.0 m³/a. The increase of water inflow into Lake Taihu is mainly concentrated in the west of Lake Taihu, which is obviously related to the water diversion along the Yangtze River. Compared with that before 2007, the water diversion volume along the Yangtze River has increased by 2.89 billion m³ on annual average, and the increase in the amount of water inflow into Lake Taihu during this period is related to this diversion. The increase of water inflow into Lake Taihu began to affect the water quality structure of “Turbid in the West and Clear in the East”. The shortening of the water exchange cycle will homogenize the water quality of east and west Lake Taihu, and the water quality of the east Lake Taihu will show a downward trend.