The gradual change of transition channel width is a common form of natural rivers. The flow structure of transition channel was measured in the physical model by Acoustic Doppler Velocimeter (ADV). It is manifested that the flow was influenced remarkably by the streamwise velocity. Secondary circulation was induced in the gradually shrinked section and in the gradually expanded section, with the circulation effect in the former greater than the latter in an opposite direction. According to the 2-D flow simulation by SMS hydrodynamic model, the coefficient of local head loss was proved to be consistent with the measured value, and the coefficient values at flow separation zone were times larger than that in other regions.