The model of slope reinforced by roots is established and the numerical simulation is carried out by using the pullout resistance of different roots length according to the principle of the axial force and the shear resistance of the woody roots system to the slope. In the process of simulation, the effects of the roots length, roots density and rainfall conditions on the slope stability are studied. The results show that the roots of woody plants can improve the shear strength of soil, increase the stability of slope and make the slope more stable. Increasing the length and density of woody plants can make the safety factor of slope larger. The roots system can still increase the safety factor of slope, and has a certain reinforcement effect on the slope under rainfall conditions.