Abstract: Underlying surface has led to the complex hydrological response of watersheds in small mountainous areas. Among them, the slope can be the basic unit of hydrological response. It is of great significance to explore the hydrodynamic features of runoff under different conditions of underlying surface on mountainous slopes. In this study, the underlying surface of the slope was selected in the typical mountain areas, the Guanshan River watershed. Multiple simulated rainfall experiments were conducted in the laboratory. A systematic investigation was made to clarify the runoff and erosion hydrodynamic features of the soil slope under different slope gradients, soil thickness distribution, and soil base permeability. The results showed that: 1) The Reynolds number, shear stress, and runoff power increased with the increase of rainfall intensity, whereas, the Darcy-Weisbach resistance coefficient decreased. The rain intensity initiated a significant increase in the mean values of Reynolds number and runoff power (P<0.05). The mean values of Reynolds number and runoff power at the rain intensity of 60, 90, and 120 mm/h increased by 130%-435%, compared with the mean values of 30 mm/h. Slope gradients shared significant positive effects on the mean shear stress and mean runoff power (P<0.05). The mean shear stress for the 15° and 25° slope gradients treatments increased 135% and 187%, respectively, compared with the 5 slope gradients treatment; The mean runoff power increased 224% and 357%, respectively. The thickness of the soil layer had significant effects on the mean Darcy-Weisbach resistance coefficient, mean Reynolds number and mean runoff power under a permeable soil base. The permeability of the soil base also exhibited a significant effect on the mean Reynolds number and the mean shear stress, when the thickness of the soil layer was thin at the top. The hydrodynamic features depended mainly on the rainfall intensity. The variance contributions to the Reynolds number reached 83.11%. The dominant influencing factors on shear stress and runoff power were the rainfall intensity and slope gradients. The sum of both variance contributions to shear stress and runoff power reached 67.64%, and 80.58%, respectively. Slope and soil thickness were the main influencing factors on the hydrodynamic parameters under a single rainfall intensity, followed by the interactions between soil thickness and slope gradients. The bottom permeability and soil thickness also presented some influence on the hydrodynamic parameters. Therefore, it is very necessary to consider the soil bottom permeability, soil thickness, and their interaction on hydrodynamic features, rainfall intensity, and slope gradients. The findings can provide some theoretical support to determine the influence of runoff and sediment yields on the complex slope surface. Some parameters can also be offered to simulate the hydrological process in small mountainous watersheds.