Abstract: Soil is easily lost in rocky desertification areas, and vegetation restoration is difficult. Selecting representative shrub species, Indigofera amblyanthy and Cassia bicapsularis, in the rocky desertification area of Guizhou as research object. Direct shear test on I. amblyanthy and C. bicapsularis root-lime soil remolded specimens using a quadruple direct shear. The effects of root diameter, normal pressure and water content on the shear properties of root-soil composites were analyzed. Results show that the existence of plant roots has a positive effect on the shear strength of the soil. The shear strength of the soil is mainly strengthened in the form of reinforcement. The effect of slope protection is 2−3 mm on the slope protection in rocky desertification areas. The normal pressure has the most significant effect on the shear properties of the composite. It affects the shear strength indirectly due to its influence on the compactness of the composite. The greater the normal pressure, the better the soil compaction and the shear resistance of the root-soil composite. The shear strength of the root-soil composite increases first and then decreases with the increase of water content. At the moisture content of 14.3%, the root-soil composite has the highest shear strength and the best soil-fixing effect. Among the 2 plants, I. amblyanthy is more able to enhance the shear strength of the soil than C. bicapsularis. This study can provide reference for the screening of slope greening and soil and water conservation tree species and the control of root diameter in rocky desertification areas.