To elucidate the hydrological evolution patterns of Lake Taihu, this study utilized compiled hydrological data from 1986–2024, employing inflow/outflow volumes and water retention time as key indicators. Mann-Kendall trend tests and other statistical methods were applied to analyze trend and periodic characteristics, identify abrupt change points, and investigate driving factors. The results shows that: (1) Both the inflow and outflow of water have shown a significant upward trend and experienced a sudden change in 2007, with main cycles at 23-25 years, 14 years, and 9 years. (2) Spatially, the spatial structure of the inflow of water has been reconstructed. The inflow and proportion in Huxi area of the lake have shown a significant upward trend, while the inflow in Zhexi area has slightly increased. The inflow from the Yangtze River to Taihu Lake has risen to the third place. The inflow in the Wuchengxiyu area and the Hangjiahu area has sharply decreased in 2009 and 2004, respectively. In terms of outflow, the WanYu River, Zhexi area, and the Hangjiahu area have shown a significant upward trend. (3) showed a significant decreasing trend with a synchronous abrupt change in 2007, dropping from 227 days annually before the change to 195 days; when accounting for peripheral water withdrawal, it decreased more sharply from 226 days to 172 days. After the 2007 change, water withdrawal caused an average reduction of 23 days in the retention time. Further analysis revealed that watershed precipitation was the primary driver in the early period (1986–2006), but after the 2007 change, engineering regulation significantly altered the inflow mechanisms in Huxi, Zhexi, and Wucheng-Xiyu areas. Consequently, total inflow around the lake (a composite of rainfall-runoff and artificial regulation) replaced precipitation as the dominant driving factor of the retention time.