Water intrusion from the mainstream into tributaries frequently occurs since the Three Gorge Reservoir(TGR)impounded. To explore the hydrodynamic characteristics of the mainstream-tributary water exchange process, field observation was carried out to obtain the monthly data(from September 2012 to July 2013), including the hydrodynamic parameters, temperature and turbidity of the Caotang River, a typical tributary in the central area of TGR. Water exchange in different impoundment periods and its influences on water environment of the reservoir were analyzed. The results show that there is a clear bi-directional flow structure in the estuary of the Caotang River and the distribution patterns of the inward and outward flow varies in different periods at the mainstream-tributary interface. The effective flow exchange in most months of a year at the estuary of the Caotang River is higher than that in other tributaries due to the unique river orientation. The water in the mainstream can intrude into the end of Caotang Bay all year round. During the high water level operating periods, inflow and outflow in the estuary shows vertical stratification and good continuity while during the low water level operating periods, horizontal circulation occurs with poor continuity.