In order to carry out a quantitative analysis and research on the main reasons for the amount of water entering Lake Taihu in Jiangsu, we conducted mutation tests on the amount of water entering the lake, construction land, rainfall and evaporation, and the amount of water diversion from the river in the past 30 years, using Mann-Kendall trend test and mutation test method, Envi5.3 and Pearson coefficient method, based on the meteorological data, remote sensing data, and water diversion data along the river from 1990 to 2019. Furthermore, taking the amount of water entering the lake as the dependent variable, we quantitatively evaluate the correlation and contribution of each factor. The results show that the amount of water entering the lake in the study area has risen to 95% in the past 30 years. The urbanization rate, meteorological conditions, and the amount of water along the river are the main reasons for its changes. Among them, human interference (urbanization rate and amount of water along the river) and natural influences (weather conditions) each account for 50%. In the future, it is recommended to strengthen the construction of sponge cities, and rationally and scientifically dispatch and use water resources along the river to reduce the impact of the increase in urbanization and external water diversion.