After the impoundment of Three Gorges Reservoir, parts of the tributaries in Three Gorges Reservoir area are influenced by the backwater of Yangtze River mainstream, giving birth to tributary bays. The water flow pattern changes from typical river pattern to lake-like pattern. Hydrodynamic conditions of the bay hence become extremely complex under the combined effect of factors like the upstream inflow and reservoir operation. In view of this, a unique flow velocity monitoring method is presented based on long-term field observation and the complex hydrodynamic condition of the bays. By using this method in Xiangxi River Bay, better flow measurement results have been obtained. It can better characterize the hydrodynamic property of water bodies of the bays under low-flow condition, and therefore proves to be an effective method.