Slope soil erosion is a complex interaction of runoff scouring, slope anti-erosion effects and ground substance supplementing capacity. A field experiment of artificial runoffs was carried out over 20° slopes of barren land (CK), fish-scale pits (Treatment YLK), alfalfa land (Treatment MXCD) and straw-mulch (Treatment JGFG), separately, to study hydrodynamics of the sheet flow scouring soil particles on the slopes and analyze it using the runoff shear stress theory, unit runoff power theory and runoff kinetic energy theory, separately. Results show (1) sediment transport rate was in a good linear relationship with both runoff shear stress and runoff power, and in a good logarithm relationship with water flow energy; critical shear stress and critical power existed throughout the erosion process; (2) with increasing volumn of the flow, velocity of the flow on the slope rose rapidly, thus leading to greater flow shear stress, unit stream power and runoff kinetic energy, and hence greater soil scouring capability of the flow, and eventually aggravation of soil erosion. On the whole the three theories demonstrated their own respective features in describing soil erosion process: the runoff shear stress theory is the best to reveal soil particles detaching process in detail, while the runoff kinetic energy and power theories are fit to describe soil erosion process on slope more simply and more accurately.