Abstract: As ecological balance and environmental protection have been increasingly recognized and valued, many developed countries have paid attentions to the ecological restoration and protection technology of slope engineering. Such technology has become the preferred solution for disaster prevention and erosion control and represented the development direction of slope protection. The shear strength of pure soil and the roots-soil composite system of two vegetation species are tested in laboratory. The two species are Astragalus adsurgens Pall. and Caragana microphylla Lam. and are planted in slope composing silty clays with low liquid limit. They are endurable to drought and barren resistance. This paper investigates the effects of different root density and moisture content on shear strength. The results show that the shear strength of the root-soil composite system increases linearly with increasing vertical pressure. Root can increase soil shear strength. The root of Astragalus adsurgens Pall. has the most noticeable effect, which is followed by Caragana microphylla Lam.As the root density increases, the shear strength increases. But, the shear strength of root soil-composite increases firstly and then decreases as the moisture increases. Plant root has little effect on the internal friction angle, while it has larger effect on the cohesion. So the increase of cohesion can greatly improve root-soil composite shear strength. The results of this test have great significance to the ecological protection technology of the slope composing silty clays with low liquid limit and ecological management of mining area in the way of plant selection and configuration.