In order to mumerically simulate 2D flow, under 6 kinds of flow ratios, at a sharp-edged, open-channel junction with 90°for channels of equal width, software Fluent with RSM turbulence model is introduced to solve the Navier-Stokes Equations. The comparison between numerical simulation sizes of separation zones and experimental data prove that the RSM model does well in the flow of small flow ratio. Thus the hybrid turbulence model is recommended, and it is suggested to use the RSM model when the flow ratio is less than 0.583, otherwise, H-L model. Furthermore, the relation diagram of separation zones’ length, width and flow ratio is plotted.