Steep slopes of Pisha-sandstone are the major sources of silt and coarse sand in the mid-reaches of the Yellow River of China. Revegetation of herbaceous plants on the slopes is one of the most effective measures to control soil erosion thereupon. However, so far little has been reported in the literature about how herbaceous plants influence soil erosion on steep slopes of Pisha-sandstone. 【Objective】This study aimed to quantitatively study effects of herbs on soil erosion on steep slope of Pisha-sandstone and hydraulic mechanism of the erosion. 【Method】 A total of ten artificial slopes (37°) were set up and planted with alfalfa plants different in spacing (2 cm、4 cm、5.6 cm、8 cm ) in Erdos, Inner Mongolia. Plot runoff scouring experiments were carried out with a constant inflow (2 L.min^-1). Runoff and sediment yields on the slopes and variation in hydraulic features of the runoff were monitored and analyzed to explore the mechanism of alfalfa controlling soil erosion. 【Result】 Results show sediment yield on the slopes planted with alfalfa occurred in the initial period of runoff and scouring period, forming a single peak. Sediment yielding rate increased with flow velocity and stream power in a power function, with determination coefficients being 0.94 and 0.68, respectively. The presence of alfalfa decreased runoff volume and changed the flow hydraulics. Flows on the alfalfa covered slopes were in a laminar and subcritical state. Increase in plant spacing resulted in greater flow velocity ( R²=0.90) and stream power ( R²=0.62), but a decrease in flow resistance ( R²=0.87). Stems and roots of alfalfa played an important role in altering flow hydraulics. Before rill formation, stems of the plants hindered the flow of runoff. However, after rill formation, apart from the stems, roots of the plants standing out in the rills inceased resistance to the flows therein. Due to the changes in flow hydraulics and soil erodibility, soil loss on the slopes decreased exponentially with alfalfa coverage ( R²=0.55). However, compared to the alfalfa coverage, plant spacing was more closely related to soil loss ( R²=0.93). 【Conclusion】 Plant spacing may reflect more comprehensively the effect of alalfa as a whole (shoots and roots) on controlling soil erosion of steep slopes of Pisha-sandstone. The study may provide a certain scientific basis for planting herbs on steep slopes of Pisha-sandstone for erosion control.