Soil erosion has been a worldwide environmental problem.Soil erosion results in not only degeneration of soil productivity, but also floods and pollution of water resources.Soil erosion and its controls have been studied extensively.Many researches revealed that the rill erosion accounts for a large amount of total erosion on slope.But, few researches have studied the mechanism of the rill erosion occurred on slope.In this study, by using runoff scouring on the definite roughness glass flume under the different slope gradients(16.5°, 20.5°, 27°) and the different flow discharges (0.2 L s^-1, 0.3 L s^-1, 0.4 L s^-1), the rill erosion mechanism were studied.The results show that the runoff on the slope moves in the form of roll waves under the given experiment condit ion.The number of roll waves and the superposing of roll wave are heavily influenced by the discharge of flow and the slope gradients.The larger of flow discharge and the greater of slope gradients, the more decrease of the number roll waves.At the same time, the scouring ex-periments were conducted on the soil bed flume under the different flow discharge(2.5 L min^-1, 3.5 L min^-1, 4.5 L min^-1, 4.5 L min^-1, 5.5 L min^-1, 6.5 L min^-1) and the different slope gradients(6°,9°, 12°).The relationship between the soil detachment and the runoff energy consumption was studied by using the law of conservation energy in this article.The soil erosion rate model was built, and a new rill erosion critical index was put forward.Based on the analyzing the data of experiment, the author got that the rill erosion occurred when the energy consumption exceed 7.38 J.