Abstract: This paper investigates the slope protection effect by vegetation and its mechanical role in increasing soil strength. It chooses six types of combinations of herbs and shrubs with their growing period of 1 a as the tested species. Built on these, the undisturbed root-soil composite systems of six types of vegetation combinations received direct shear tests to evaluate their contribution in increasing soil strength and to analysis their roots distribution characteristics. The results exhibit that for the six types of vegetation combinations, most of their roots distribute in a range of 0 to 30 cm beneath the ground surface, with their roots distribution type exhibiting a linear or exponential declining trend. The mass ratio of herb roots to shrub roots exhibit an initial decreasing and then increasing trend with soil depth increasing, which can be described with a quadratic function. In the depth of 0 to 30 cm beneath the ground surface, the cohesion of the root-soil composite systems of six types of vegetation combinations increases by 2.47% ~70.96%, comparing with that of soil without roots. The result indicates the roots of the six types of vegetation combinations exerts a significant role in increasing soil cohesion. Of these vegetation combinations, the vegetation combination of Medicago sativa L.+ Elymus nutans Griseb.+ Puccinellia distanx(L.)+ Poa pratensis L.+ Caragana korshinskii Kom exerts a dominant role in increasing soil cohesion compared to other vegetation combinations. The results of this study have practical guiding significance and theoretical research value for the ecological restoration and vegetation slope protection project in Xining Basin and its surrounding areas.