Numerical simulations for open-channel combining flows with equal channel width were performed with a three-dimensional numerical model with RSM and VOF method. The results were analyzed for 21 cases, in which seven junction angles and three discharge ratios were combined. The separation zone is defined with the isoline method of longitudinal velocity. The sizes and the three-dimensional shape of the separation zone were analyzed. Meanwhile, the streamwise distribution of the secondary-current intensity downstream the junction was summarized in terms of the sectional vorticity. The results reveal that as the discharge ratio increases or the junction angle decreases, the horizontal sizes of the separation zone decrease, and its shape becomes long and narrow. The averaged depth of separation zone decreases while the discharge ratio or the junction angle increases. The intensity of the secondary currents downstream of the junction gradually reduces while the discharge ratio increases. The impact of the junction angle is significant near the junction and gradually reduces along the streamwise direction.