Hydrolic retention time in an aquatic environment of lake water has long been of interest to biological limnologists because it is very important for determining lacustrine ecosystem health and sensitive to pollution threats. To evaluate the retention time of Lake Taihu, three scenarios are set up according to the measured data of hydrological, meteorological, boundary conditions of water quantity and quality around the lake in 2010, in which the first is the real scheme, the second is the inflow flux multiplier scheme around the lake, and the last is Wangyu River inflow flux multiplier scheme. EcoTaihu Model is adopted to simulate the nutrients conditions in Lake Taihu and its subzones. The results show that under the hydrological, meteorological and the boundary conditions in 2010, an appropriate hydraulic retention time of Lake Taihu is between 150 days and 160 days.