In order to study the hydrodynamic mechanism of soil erosion on earth-rock mixed colluvial deposit slopes, scouring tests were conducted in the laboratory. In the tests, four the gravel contents (mass content) (0, 20%, 40% and 60%), four flows (2, 4, 8 and 16 L/min) and four slope gradients (10°, 20°, 30° and 40°) were set. Results showed the following: (1) soil erosion rate decreased with the increasing of gravel content when the slope gradient or flow was low, while when the flow or slope gradient increased, erosion rate first decreased and then increased with the increasing of gravel content; erosion rates of different earth-rock mixtures could be expressed by power function of slope and flow. (2) Flow shear stress, stream power and unit stream power were all linearly related to erosion rate, while by comparison, shear stress and stream power were the optimal hydrodynamic parameters to simulate the erosion process on the earth-rock mixed colluvial deposit slopes. (3) Soil erodibility parameter and critical erosion force both increased with the increasing of gravel content. These results of this study could provide a reference for further understanding the erosion mechanism of colluvial deposit.