At present, the theoretical formulas for determining the width of waterway regulation lines were mostly solved by establishing a certain relationship between the flow and sediment conditions before and after the project. Many parameters were involved in these formulas, and in the solving process, it was usually assumed that value of part parameters in the formula is the same before and after the project. However, some changes may occur before and after the project, affecting the accuracy of the width of regulation lines. In order to avoid errors caused by this assumption, a new method to determine the width of waterway regulation lines was proposed. Based on the mechanism of flow movement, sediment movement and river bed topography, two determining methods for the width of waterway regulation lines under continuous scouring condition and sediment transport equilibrium condition were put forward by theoretical study, and the proposed methods were verified by practical projects with good regulation effect. The results show that the proposed methods have good engineering results, and compared with most existing methods, the width of the regulation lines determined by the new method is the closest one to the actual value of engineering project, which can provide a good theoretical support for waterway regulation projects.