Abstract: Based on the topographic features and data of sediment particle size of bed load at the fluctuating backwater reach of Liujiaxia Reservoir,the streamwise variation and variation process of median diameter of bed load during silting and scouring periods of the fluctuating backwater reach are analyzed,and it can be seen that the turning point for the sediment diameter variation from coarse to fine is located at the outlet of Shigou Gorge of the fluctuating backwater reach.