In some physical model experiments, it is necessary to use distorted models. It is difficult, however, to build in an optimal degree of distortion into the models to ensure a closer degree of similarity between the model and the prototype. Three-dimensional (3D) numerical simulation is used in this paper to model experiments with a straight channel under different distorted scales. The calculated results of stream-wise, lateral and vertical velocities and sediment concentration along the vertical direction are shown by comparing the deviations of the velocities and sediment concentration between the distorted model and a normal one. Generally the discrepancy between the distorted model and the normal in stream-wise velocity is acceptable, while in vertical and transverse directions, the velocity shows differences. Concerning sediment concentration and channel bed deformation, the effect of the distorted scale is mainly related to two different similarity criteria. The similarity ratio between the turbulence diffusion velocity and gravity settling velocity can reproduce better results of sediment concentration along the vertical direction. The better bed deformation results come, however, from the similarity ratio between the averaged flow velocity and gravity settling velocity.