To explore the change characteristics of rill flow velocity on the soil-rock mixed colluvial deposit, the scouring experiment was carried out. Four flow rates (2, 4, 8, 12 L/min) and four gravel mass contents (0, 10%, 30% and 50%) were set up. The change characteristics of rill flow velocity on the soil-rock mixed colluvial deposit were analyzed, and the response of flow velocity to flow rate and gravel content was studied. The results showed that the velocity decreases exponentially with scouring time, and the velocity attenuation trend of pure soil is larger than that of gravel-containing slope. The velocity of flow increased as a power function with the increase of flow velocity and decreased as a logarithmic function with the increase of gravel content. The effect of flow velocity and gravel content on flow rate was significant, and the effect of flow rate was greater than that of gravel content. Flow rate can be expressed by binary logarithm of flow rate and gravel content (NSE=0.78). The research results deepen the understanding of hydrodynamic process of rill erosion on colluvial deposit slope.