Due to the difficulty of extracting runoff depth (1~5 mm) and hydrodynamic parameters (velocity, depth, etc), it is difficult to study the water flow structure, energy dissipation and sediment transport of runoff erosion on slope surface. Based on the principle of hydrodynamics and similarity theory, the flow similarity process of overland sediment flow on slope surface was simulated by enlarging the depth of runoff with non-conventional scale model. The results were as follows:(1) When the water depth of overland sediment flow was enlarged by 2.5 times and sediment content was 10~320 kg/m³, the errors of surface line (resistance), velocity and erosion topography from the prototype and model of overland sediment flow were 0~0.1%, 0.1%~5.3% and 0.9%~4.9%, respectively, all the errors of which were within the allowable ranges. The prototype and the model fitted the similarity transform relationship, such as geometry and motion. (2) When the water depth was 0.5~1.25 cm, the overland sediment flow was turbulent, the vertical distribution of velocities of the prototype and the model fitted the logarithmic distribution, which could be expressed by the same equation. (3) The non-conventional scale model could be used for the further studies of hydrodynamic parameters extraction, flow structure, energy dissipation and sediment transport of the overland sediment flow.