In order to define the flow characteristics of the water sediment interface of the wavy river bed, the river bed is summarized as a triangular river bed, and the numerical simulation is carried out by using the software of COMSOL Multiphysics. Firstly, the sediment depth of the river bed is determined according to the relationship curve between the sediment depth and the depth of the undercurrent positive exchange area. Five kinds of triangular river beds are set up to study the flow characteristics under two working conditions. The parameters studied include the velocity of water sediment interface and the exchange capacity of undercurrent. The results show that the shape of the triangular riverbed has a significant effect on the area of upflow and downflow, and the particle velocity at the interface of water and sediment. The exchange volume of upflow and downflow is approximately the same, and the exchange volume of undercurrent will change with the shape of the triangular riverbed.