The gradual characteristics of variations in transverse width often occur in mountain river because of the effect of geological conditions and river evolutions. This will influence flow and sediment transport due to the variations of flow structures resulted from the river width changes. A flume experiment that was conducted to explore how flow characteristics perform with the varied channel width, showing the connection of the flow parameters, flow rate and channel width. For example, the longitudinal water depth and velocity change along with the transition width of flume. On the other hand, a depth-averaged two-dimension model was proposed according to series of methods such as body-fitted coordinate, finite volume method, SIMPLER algorithm. The numerical results agreed well with the experimental observation in a flume, which showed that the model could reasonably simulate gradual channel transition flow. In addition, the validated numerical model was applied to simulate flow with different flow rates with the same imposed boundary condition in order to further analyse the effects of channel width variation on flow characteristics.