The complex underlying surface conditions in mountainous areas lead to the complexity and high spatial heterogeneity of runoff and soil erosion processes in mountainous areas. As the basic unit of hydrological response in mountainous watersheds, it is of great significance to study the hydrodynamic characteristics of runoff and erosion under different underlying surface conditions. In this paper, combined with the underlying surface characteristics of the slope in a typical mountain watershed, Guanshan River watershed, multiple simulated rainfall experiments were conducted in the laboratory, the runoff and erosion hydrodynamic characteristics of the soil slope under different slopes, soil thickness distribution and soil base permeability were studied. The results showed that: There were significant differences in the mean runoff power under the design rain intensity conditions (p < 0.05). The mean runoff power at the design rain intensity of 60, 90 and 120 mm h−1 increased by 130%, 276%, 366% and 171%, 328% and 435%, respectively, compared with that of 30 mm h−1. The mean value of shear stress and runoff power increased significantly with the increase of slope (p < 0.05). The mean value of shear stress at 15 and 25° increased by 135 and 187%, respectively, compared with that at 5°. The mean runoff power of 15 and 25° increased by 224 and 357%, respectively, compared with that of 5°. There is a good liner positive correlation between runoff rate and shear stress, as well as with runoff power. The correlation coefficient of runoff rate and shear stress is basically more than 0.7 (except for TBI-25, 0.43). The correlation between runoff rate and shear stress is both affected by slope, soil thickness distribution and water permeability of soil layer bottom. The correlation coefficient of runoff power and runoff rate is above 0.85 (except TBI-25, 0.54), the correlation between runoff power and runoff rate is mainly affected by the slope. Under continuous rainfall conditions, the linear correlation between shear stress, runoff power and erosion rate is significant when the rill erosion is considered only under two rainfall conditions. When the soil layer thickness distribution is thick at the top, the linear correlation coefficients between shear stress, runoff power and erosion rate are all above 0.66. The research results can provide scientific basis for revealing the mechanism of runoff and sediment yields on mountain slope under complex underlying surface conditions and improving the research on hydrological processes in mountain small watershed.