The reaches of the Yangtze River below Nanjing are mainly the branch river type. The movement of water and sediment，and riverbed erosion and deposition are complex by the combined action of runoff and tide. Taking the Yizheng waterway of key navigation obstructing shoal in 12.5 m deep water channel below Nanjing as an example，we analyze the evolution characteristics of this waterway before and after impoundment of the Three Gorges Reservoir according to the existing research results and the recently measured hydrological，sediment and topographic data，and discuss the influence of water and sediment changes on riverbed erosion and deposition after the impoundment of Three Gorges Project. The results show that after the impoundment of the Three Gorges Reservoir，the riverbed erosion and deposition are different from those before impoundment. Before the impoundment，the Shiyezhou left branch scours，and the right less deposition；after storage，both of them as scouring. Before the impoundment，the flood has a great influence on the pattern of the channel beach. After the water storage，the flood water is affected by the reservoir，and the flood decreases，the influence of the water and sand on the evolution of the beach is weakened.