Mountain torrents occur frequently in China,and people pay more and more attention to the factors of sediment disaster.The flume test is carried out to simulate the slope change and large amount of sediment transport in mountainous rivers.Adopting the single factor experiment，we study the variation law of water level caused by large gradient and strong sediment transport.The results show that carrying a large amount of sediment,the rapid deposition of sediment contributes to the slowing down of local slopes,the decrease of flow velocity and the rise of water level. On the other hand,the resistance of sediment itself leads to the rise of water level.Thus the water jumps in some sections happen alternately,and the flood level increases significantly.The interaction between sediment and water results in that the water level at the junction front is far higher than the clear water depth.The research reveals that the higher the discharge,the larger the sediment transport rate or the larger the upstream channel gradient is,and then the higher the water level at the junction front will be.With the retrograde development of sediment deposition,the abnormal surge of water level will not only occur near the slope junction,but also spread upstream,causing the rivers to overflow and flood around.