On the basis of scouring experiments indoors, the mechanism of the drop pit'genesis and development during the process of slope erosion is analyzed, and the relationship among headcut, bank landslip and downcut has been expounded. The variation regularity of the runoff and the sediment yield in each discharge was studied, and the relationship among the sediment, the runoff and the proportion of the rill area was obtained. Analyzing each erosion stress on the slope during the scouring, we get the runoff shearing sediment equation. The study shows that the sediment yield from runoff shearing take on a good positive correlation with the average runoff shear stress, and the critical shear stress of the runoff, by which the soil grain can just be dispersed, is 0.258 N/m².