To study the mechanical effect of shrub plants on slope protection and to investigate the friction characteristics of root-soil interface, the roots of Indigofera amblyantha were sliced and the root surface roughness of this kind of plant were calculated. In order to evaluate the influence of different root diameters, soil moisture content and soil particle size on friction characteristics of root-soil interface, pull-out friction tests and shear friction tests were conducted on the root - soil composites with the particle size of 0 ~ 0.5,0.5 ~ 1.0,1.0 ~ 1.5 and1.5 ~ 2.0 mm which prepared by sieving and remolding. The results showed that there was no significant difference in root surface roughness of I. amblyantha with the gradual increasing of root diameter, the pull-out shear stress of root system was concentrated in 17.36 ~ 32.76 kPa, and the friction coefficient of root-soil interface varied between 0.10 and 0.20. The pull-out shear stress of roots and the friction coefficient of root-soil interface in soil-root composites presented a downward trend with the increase of soil moisture content and soil particle size. When soil particle size and soil moisture content were small, the effects of root morphology on the pull-out shear stress of I. amblyantha roots and the friction coefficient of root-soil interface were more significant. These results were great guiding significance to analyze the influence factors of root-soil interface friction characteristics, and use shrub plants to improve the stability of slope soil structure and prevent soil erosion and other geological disasters.