Abstract: In order to provide scientific basis for loess plateau water erosion forecast model, the hydrodynamic characteristics of runoff in the process of rill erosion were studied by using runoff scouring experiment under the different steep slope gradients(21°, 24°, 27°) and the different flow discharges(6.5, 8.5, 10.5 L/min). The results show that the Reynolds number of the slope flow varies between 798 and 4620 in the erosion process and their changes ranges increase with time. The Froude number of slope flow is larger than 1 in the whole experiments, which indicates that the slope flow belongs to the supercritical flow condition. Roughness coefficients of rills flow increases with the increase of Reynolds number and are easily affected by the slope gradient. During the scouring and erosion of runoff on slop, flow velocity increases first, then decreases and then decreases with the prolongation of scouring time and the variation of scouring patterns.