The hydrodynamic characters of rill flow in the process of rill erosion in purple soil were studied by using runoff simulated experiment under the different slope gradients and the different flow discharges.The results show that the velocity of rill flow on purple soil slope increased according with the increase of runoff,but with the same runoff flux,the velocity of rill flow has no obvious trend.The Reynolds number of rill flow varies between 500 and 4 600 in experiments and it increases quickly with the increase of flow flux on the same slope.The Froude number of rill flow is above 1 in experiments and it belongs to the supercritical flow.The roughness coefficient affected by the slope gradient and flow flux,is closely related to the Reynolds number.It decreases with the increase of Reynolds on gentle slopes,and increases with the increase of Reynolds on steep slope.