In order to explore the influence of gravel content on rill erosion of colluvial deposit slope, the combination souring tests were conducted. This paper studied the morphological characteristics of rill cross section during scouring of four kinds of soil-rock mixed colluvial (gravel mass ratios of 0, 10%, 30% and 50%, respectively) with different scouring flow rates (2, 4, 8 and 12 L/min, respectively) under slope gradient of 30°. The results showed that:(1) With the increase of scouring time, the increasing speed of depth was greater than that of width, which made the shape of rill cross section develop toward narrow and deep. (2) The depth of rill cross section was large on the top of slope, which was similar to the "V" shape, and the erosion was severe. The depth of rill cross section was small on the bottom of slope and the erosion was relatively mild. The morphological index η of rill cross section fluctuated as the increase of slope length. (3) With the increase of gravel contents, the depth of cross section decreased first and then increased. The depth of rill was shallowest on the slope with 10% gravel content. The morphological variability of cross section was maximum on the slope with 10% gravel content. The free surface began to develop at the depth of 10~15 cm, and the soil with low gravel content was more conducive to the development of the free surface.