Abstract: The regulation of water volume and discharge during irrigation is a dynamic process. The surplus water caused by improper regulation of discharge is a main factor of water loss in irrigation districts. In order to precisely regulate the discharge of channels, the problem of unsteady flow caused by discharge variation due to regulating diversion sluice during the water distribution process on main irrigation channels was researched. The mathematic model was described by the Saint-Venant Equation; the characteristics method which is in a rectangular characteristics difference grid and has two order precision format was applied in numerical simulation. Transition graphs of water level and discharge in the upper and downstream boundaries of the simulation channel under conditions of different regulating discharge and time and their relation to different initial discharge, water level and regulating patterns are also given.