Abstract: In-situ direct shear tests on the undisturbed soil and four root-soil composite systems of camphors were conducted at the foot of the Nanwang Mountain. Shear strength and displacement of them were compared and mechanical effects of the root systems on soil reinforcement and slope protection were evaluated.The results showed that:1) the shear strength and residual strength of soils were reinforced by root systems and the increased amplitude of the residual strength was higher.2) Compared with the soil without roots,the displacements in the intensity peak of four soils with roots were significant.The latter could withstand greater deformation before cutting break and the root systems increased the ductility of the soil.3) The curves at the strain-softening stage of four soils with roots were more moderate than that of soil without roots.The reducing amplitude of shear strengths of the former after reaching the peak was reduced and the soil with roots could remain a higher strength when a larger strain appeared,so the ductility of the soil with roots was increased.4) The increment of shear strengths was correlated to total root length and root area ratio on the cut section,respectively.