In order to study the effect of floodplain on the flow characteristics and pollutant transport of river confluence, a three-dimensional hydrodynamic and pollutant coupled numerical model is established to simulate and analyze the effects of floodplain with different heights and widths on the flow structure and transverse mixing characteristics of pollutants in the confluence in this study. The results show that the vertical momentum redistribution of the lateral inflow, which is caused by the floodplain, has a significant influence on the structure of the twin-helical flow, the transverse advection range of pollutants, and the position, morphological characteristics and width of the mixing layer. The intensity and range of the secondary flow increase with the increase of floodplain height. The widening of the floodplain makes the secondary flow deviate from the left bank and its intensity increases. In addition, the mixing degree of pollutant is closely related to the secondary flow, i.e., theintensity, location and range of secondary flow;the higher intensity, the largerrange and the closerpositionto the mixing layer can result in the faster the material mixing rate.