A particular type of gully erosion with a steep collapsing wall, locally called “Benggang”, is widely distributed in the granite-covered areas of tropical and subtropical South China. These gullies develop fast and erupt suddenly, cause great damage. “Benggang” is generally composed of upper catchment, collapsing wall, colluvial deposit, scour channel, gully mouth, and alluvial fan. Colluvial deposits are materials accumulated at the foot of the collapsing wall. They were originally part of the collapsing wall and surface of mountain slopes and fell apart under hydraulic pressure and gravity. As a type of disturbed soil, colluvial deposits feature high contents of gravel, sand, and loose materials, so that they are often weak in structure, low in cohesion, poor in stability steep in slope (generally varying between 20 and 40°) and high in erodibility. Furthermore, rainfall splash erosion and runoff scouring erosion occur easily because in addition to the above listed properties, they lack vegetation cover and organic matter. As a result, erosion of colluvial deposits triggers redistribution of materials from the collapsing walls, generating large volumes of sediments, which in turn aggravate the harm of Benggang. Particle size composition of sediment is an important factor of soil erosion. The study on peculiarity of erosion sediment will be of great significance to illustration of mechanism of the erosion on slopes of colluvial deposits. However, so far few reports are available on erosion of colluvial deposits. To study properties of erosion sediment from colluvial deposits, rainfalls were artificially simulated, with intensity ranging between 1.00 and 2.33 mm min^-1 over a plot 5 m² (5 m×1 m) in area and 20~40°in slope. Results show the following: (1) Changes in sediment particle size composition and enrichment rate: As fine particles are washed preferentially by rainfall, after each rainfall event, the mean enrichment rates of clay, fine silt, course silt, and fine sand are all higher than 1; With rising rainfall intensity and slope gradient, both the content and enrichment rate of coarse particles increase in the sediment, making it coarser in texture. (2) Processes of sediment generation during a rainfall event: At the initial stage of runoff generation, as a result of splash erosion caused by rain drops, fine particles display a process of rapid increase, while coarse particles do a reverse one, which is especially obvious under rainfalls 1.00 and 1.33 mm min^-1 in intensity. At the interrill erosion stage, interrill flow is far from strong enough in transport capacity, and carries only fine particles, like clay, fine silt, coarse silt and fine sand, of which the enrichment ratios all exceed 1; but the formation of crust reduces loss of soil particles. After rills form on the slope, scouring capacity of runoff enhances, so content of coarse particles, especially coarse sands and gravels, increases while that of fine particles decreases correspondingly, and then particle size composition gradually becomes stable and approximate to that of the original soil. So, with rainfall going on, particle size of erosion products turns gradually from fine into coarse and levels off in the end. During rainfalls, high in intensity, interrill erosion - rill erosion manifests “overall transport” of original soil. (3) Mean weight diameter (MWD) of sediment: The sediment MWD of rill erosion is greater than that of interrill erosion; which once again shows that sediment becomes coarser in particle size after rill erosion starts. Sediment MWD increases with rising rainfall intensity. Sediment MWD of inerrill and rill erosion varies with slope gradient and comes across a critical gradient, around 30°～35° under rainfalls 1.00 or 1.33 mm min^-1 in intensity,, but no such phenomenon was observed during rainfalls with intensity either higher or lower. Under rainfalls 1.67, 2.00, and 2.33 mm min-1 in intensity, MWD of rill erosion sediment is higher than that of the test soil (1.96 mm), on slopes higher than 30° in gradient, which demonstrates that at the late stage of a heavy rainfall event, erosion sediment is coarser than the original soil in particle size. Therefore, MWD varies with rainfall intensity and slope gradient as well, and particles are selectively transported in light of particle size. Dual-factor variance analysis show that rainfall intensity is higher than slope gradient in effect on MWD of rill-interrill erosion sediment.