Mountain rivers often carry a large amount of sediment in the process of flash floods and cause severe deformation of the riverbed. In particular, the riverbed of the wide and narrow rivers is adjusted sharply and the water level rises sharply, which can easily increase the risk of flood inundation in the widened rivers. The study of water and sediment transport and riverbed deformation in the evolution of mountain torrents is of great significance for flood control and disaster mitigation. In this paper, two-dimensional numerical experiments of water and sediment dynamics are used to explore the process of water and sediment transport and river bed deformation in the wide and narrow reaches under the conditions of upstream sediment changes. The results show that: when there is sufficient sand from the upstream, the flood carries the sediment in the widening section and scours the narrowing section, and the river bed is lifted up in the process of sediment transport. The main reason is that the saturated sediment transport of the channel leads to the overall siltation of the river bed, which leads to the steep rise of the upstream water level, the increase of the water surface gradient, the increase of the shear stress of the bed surface, and the increase of the sediment transport rate of the river channel, and the increase of the bed surface fluctuation of the wide and narrow reaches. The morphological resistance of the bed surface causes the upstream water level to rise; when the amount of sediment is small, the river bed is mainly scoured, the water level drops, and the water surface gradient tends to slow down.