Abstract：Four shrubs planted in PVC pipes in the testing areas are studied；then direct shear tests on the undisturbed soil and root-soil composite system are conducted. By comparing the shear strength indexes of the soil without root and four kinds of soils with roots，the mechanical effects of the root system of four shrubs with one-year growth period on slope protection can be evaluated. Comparison between the shear strengths of four- shrub root-soil composite systems is made to evaluate the enhancement effect of the systems for slope protection. The results show that：(1) when the root and water contents of the four-shrub root-soil composite systems keep constant，the shear strength increases linearly with increasing vertical pressure，indicating that the direct shear strength of the root-soil composite systems agrees with Coulomb law；(2) the cohesion of the root-soil composite systems is obviously larger than that of soil without roots，and there are no apparent changes in the internal friction angle；(3) compared with the cohesion of soil without roots，the increasing amplitudes of the cohesion of Atriplex canescens，Caragana korshinskii，Zygophyllum xanthoxylon，Nitraria tangutorum are 76.3%，62.7%，45.8% and 22.0% respectively. The relations between the shear strength and displacement of the root-soil composite systems show a linear relationship at the beginning stage of the shear process，a rounding curved relationship at the end of the shear process，and an approximate horizontal relationship when shear failure. The shear stress of the root-soil composite systems increases obviously with increasing vertical pressure，in which the increasing rule of shearing strength agrees with linear relationship at the beginning of the shearing process，basically linear at the beginning and nonlinear when approaching failure. According to the strengths of the four-shrub root-soil composite systems，the mechanical effects of the four one-year growth period shrubs for slope protection are as follows：Atriplex canescens＞Caragana korshinskii＞Zygophyllum xanthoxylon＞Nitraria tangutorum.
