Abstract: Hydraulic calculation of steady uniform flow in trapezoidal compound open channels was studied. Based on the force balance of water in each sub-section, the average velocities of the main channel, side slope and floodplain were deduced, and the lateralmomentum exchanges between the sub-sections were expressed by using the apparent shear stress. Toverify the model,seven groups of UK-FCF measured data with a relative water depth between the flood-plain and the main channel varying from 0.057 to 0.4 were adopted for comparison. The result shows that the calculated velocity is larger than them easured data when relative water depth is small, while it is smaller than or close to the measured value for cases with a larger relative water depth. The influence of the apparent shear stress on the calculation of velocity on the flood-plain is not obvious, while it is much greater on the main channel. The three-stage model is compared with Liu's two-stage model, and the conclusion is that the former can give a better prediction for a three-stage trapezoidal compound channel. Finally, the apparent shear stress is calculated and compared with the measured data, and the result shows that the values of momentum transfer coefficient adopted are appropriate.