Abstract: A analytic method was advanced for determining the relationship of sediment concentration change with rill length, from the relationship of the detachment rate of eroding rill with sediment concentration and the derivative function of detachment rate. The computed results from this analytic method was compared with those obtained directly from experiment, for the purpose of validating the method. And statistic analysis of the results under different slope and flow rate conditions showed very high correlative coefficients, with the lowest square correlative coefficient of 0.81. The results imply the validation of the analytical method advanced and that of the detachment rate function as related to sediment concentration in the flowing water, advanced in the related research.