An analytic method for determining detachment rate of concentrated flow in erosion rills was advanced in the paper.By making use of the functional relation of sediment load with rill length and the corresponding regression parameters obtained from simulated erosion experiments this method estimated functional relations of detachment rates of concentrated flows with rill length.Then relationship between detachment rate and sediment load in the flowing water was derived from this function.Comparison of the detachment rates thus computed with those directly estimated on the basis of experimental data was conducted to prove feasibility of this method.