There is limited research on the influence of near-surface coverage of flexible vegetation on reducing runoff and erosion yields, as well as on regulating the hydraulic characteristics of surface runoff. Based on the principles of fluid mechanics and hydraulics, this study conducted indoor drainage experiments under five coverage levels, five slopes, and three flow rate combinations. The erosion patterns and hydrodynamic characteristics under flexible vegetation cover conditions were systematically investigated. The results indicate: (1) The distribution of erosion volume initially rises and then falls with increasing slope, with flow rate having a more significant impact on the erosion process at higher slopes. (2) As the coverage of flexible vegetation increases, erosion volume decreases initially and then increases, with 50% coverage as the turning point. (3) The average flow velocity (v) decreases with increasing coverage, and the decreasing trend becomes less steep as coverage increases. The variation trend of the Froude number (Fr) is similar to that of the average flow velocity. The resistance coefficient (f) is linearly correlated with coverage, and with increasing slope, the value of the resistance coefficient gradually decreases. Reynolds number (Re) is more sensitive to soil erosion than Fr, v, and f. (4) With increasing vegetation coverage, morphological shear force increases (particle shear force decreases), and morphological shear force plays a decisive role in the variation of total shear stress, which is positively correlated with vegetation coverage. This study lays a theoretical foundation for understanding the erosion patterns of vegetated slopes, and promotes the extension of open channel hydraulics theory in slope surface flow.