In order to promote the quantitative study on mechanism of root-soil reinforcement, three kinds of common herbs including Paspalum notatum, Eremochloa ophiuroides and Bermudagrass were selected and artificially planted based on slope investigation and plants selection, and the direct shear test for remolded root-soil composite of these three herbs were designed and implemented on the basis of physical properties of undisturbed root-soil composite, such as dry density, moisture content and root content of quality in different depths. The results showed that: the shear strengths of three undisturbed root-soil composites rose gradually with the increasing of the root content under different vertical pressure; the cohesions and internal friction angles of three undisturbed root-soil composites decreased with the increasing of depth. Under the same condition, the effects of three plant roots on the cohesion were sorted as follows: Bermudagrass> P. notatum> E. ophiuroides，while the effects on the internal friction angle were sorted as follows: E. ophiuroides > P. notatum > Bermudagrass. The effects of plant roots system on shear strength were not only affected by plant species, but also related to the dry density and water content of soil, and the increase of dry density had a significant influences on the cohesion, but had little effect on the internal friction angle. The most suitable moisture content for the three kinds of disturbed root-soil composite was about 25%. It was difficult to explore the influences of the root content on the shear strength index in the direct shear test of remolded root-soil composite.