With the demand of ecological environment protection,research on slope protection engineering by vegetation has received extensive development.However,few quantitative works have been done for the root-soil interaction and the effects of root reinforcement to slope due to the randomness and complexity of the root distribution.The mechanism of root reinforcement to soil and the shearing model of root-soil composite medium are discussed.The contribution of root reinforcement to shear strength is considered to have the characteristics of cohesion,and the effects of root reinforcement to slope are analyzed by numerical simulation.The safety factors of slope are computed in several kinds of cases.The results of this study may provide theoretical foundations to the deeper research works such as assessments of root reinforcement, selections of vegetation and the corresponding arrangement.