The overland flow plays an important role in the process of soil erosion in loess areas, the study of overland flow dynamics and slow-flow mechanism under vegetation coverage is fundamental for understanding the cause of soil erosion. Here, flume experiments of slopes of 12° under different combinations of nine vegetation coverage and seven flow rates were carried out to analyze the variation of flow velocity and slow-flow mechanism. The results show that the overland flow velocity generally decreased with the increase of the vegetation coverage, whereas the slow-flow coefficient first increased then decreased with the increase of the coverage. When the vegetation coverage was larger than 47.66%, the slow-flow coefficient decreased significantly with the increase of the coverage. So, 47.66% was the critical value of the coverage, the changes of the slow-flow coefficient with those of the Reynolds number under high or low vegetation coverage followed different patterns around this tipping point. Finally, the formulas for calculating the velocity and slow-flow coefficient under the simulated vegetation coverage were then derived. The verification result shows that the formulas have high accuracy and can provide theoretical reference for the prediction of soil erosion.