[Objective] The hydrodynamic properties of red soil slopes under varying surface roughness, slope, and flow rate were analyzed in order to provide scientific basis for a better understanding and prediction of sheet erosion on red soil slopes. [Methods] A study was conducted to simulate red soil slopes with varying surface roughness by using different particle sizes (0.25—1, 1—3, 3—5, 5—7, and 7—10 mm). The experiments involved four flow rates (2, 4, 8, and 16 L/min) and four slopes (5°, 10°, 15°, and 20°). Photogrammetry based on the structure from the motion technique and the electrolyte tracing method were used to determine the surface roughness and flow velocity on the slope. The collected data were used to calculate the surface roughness and hydrodynamic parameters under each condition. [Results] The mean flow velocity varied from 0.022 to 0.531 m/s under the experimental design conditions. The Reynolds numbers ranged from 63 to 1 155, Froude numbers ranged from 0.1 to 4.1, and the resistance coefficients were within the range of 0.13 to 68.86. The surface roughness was positively correlated with the resistance coefficient (p<0.01) and negatively correlated with the Reynolds number (p<0.05), Froude number, and flow velocity (p<0.01). The slope was positively correlated with the unit-width discharge of overland flow and the Froude number (p<0.01). The width discharge was positively correlated with the Reynolds number, Froude number, and average flow velocity (p<0.01) and negatively correlated with the resistance coefficient (p<0.01). [Conclusion] Increased surface roughness augments the resistance to overland flow and reduces the influence of flow inertia, serving as a crucial factor in determining the mean flow velocity, Froude number, and resistance coefficient. The unit-width discharge primarily influences the Reynolds number by altering the water depth and flow inertia of the overland flow. Compared with the surface roughness and unit-width discharge, the slope had the least impact on the mean flow velocity, Reynolds number, Froude number, and resistance coefficient. The relationship of the mean flow velocity and Froude number with the surface roughness, unit-width discharge, and slope can be described well by the power function. Similarly, the Reynolds number and resistance coefficient exhibited a strong power function relationship with the unit width discharge and surface roughness.