An experiment of debris flow was carried out to study the influence of grain size distribution on starting progress mode by using the self-developed small-scale model flume under the condition of artificial rainfall. By using coarse sand (1~2 mm) and fine sand (0.075~0.25 mm), the model was made according to the quality ratio. Model test study finds that, with fine content increasing from 20% to 100%，failure pattern changes from layered and clumpy slide to fluidized flow. By using GeoDog software to analyze displacement and a pore pressure gauge to measure pore pressure changes, we have found sliding plane developing in layered and clumpy slide, and pore pressure decreasing with crack generation, and increasing with water seeping into crack. There is no crack development in fluidized flow, and pore pressure momentarily drops to a small value when damage occurs. The average volume of the starting for fluidized flow is bigger than that for layered and clumpy slide. The formation mechanism for layered and clumpy slide is that water penetrates quickly, causing slope foot damage, and the slope collapses for losing support. For fluidized flow, the permeability coefficient is small, the soil of the back is highly saturated, and liquid causes the slope to decline rapidly. Based on the indoor model test of rainfall-induced debris flow, the formation mechanisms of debris flow modes were summarized.