The interaction of flow occurs between the floodplain and the main river channel after the flood in the composite river channel overflows, which will form momentum exchange at the junction of the river channel and the floodplain. This will lead to great errors if the Manning formula is directly used for hydraulic calculation. The average vertical velocity of different composite river channels is calculated by the CES (Conveyance Estimation System) model and this model is verified by using the experimental data from British Science and Engineering Research Council flood channel facility (SERC-FCF), and then the analysis of the flow characteristics of different compound river channels is carried out in this study. Finally, by comparing and analyzing the accuracy of the CES model with that of empirical methods (such as wet perimeter correction method and cross-section segmentation method) and theoretical methods (such as the Xie Hanxiang method, Liu Peiqing method and coordination method) in calculating the discharge, the superiority of this model for hydraulic calculations of the compound river c hannels is demonstrated. The analysis results show that the distribution of the average vertical velocity given by the CES model method is in good agreement with the measured data, and it also accords with the flow characteristics of the compound river channels. Moreover, the CES model, compared with other models, is more suitable for the discharge calculation of the compound river channels with arbitrary shape and has higher accuracy.