Abstract: In order to explore the change law of flow velocity,coarsening gradation and scouring depth of mountain-stream pebble riverbed after scouring by clear water,based on the flume generalized model test,three sets of flow,slope,and non-uniform sand sediment were set to conduct a preliminary study on the change process of the riverbed. The research results show that under different working conditions,the line velocity and scouring depth are unevenly distributed along the groove width,and show an increasing trend from the side wall to the groove center. When the flow rate is constant and the slope increases,the line velocity is inversely proportional to the water depth. When the slope is constant and the flow rate increases, the line velocity is proportional to the water depth. The dimensionless parameter width-depth ratio is introduced to obtain the line velocity distribution formula suitable for different working conditions. With the increase of flow and slope,the degree of riverbed coarsening andscouring depth increase. Considering the degree of concealment of coarse particles to fine particles,the calculation formula of bed surface coarsening gradation after flushing with clear water is optimized. The calculated value of the formula is compared with the measured value,and the error of the result is small. The research results can reasonably predict the evolution trend of the river course,which provide an important basis for the safety design of the project,and are of great significance to reducing project investment,flood control safety,waterway transportation and water supply balance.