In order to systematically study the mechanical effect of shrub roots on soil consolidation and explore the friction characteristics of soil to roots, taking four different soil particle sizes (<2.0, 1.0~2.0, 0.5~1.0 and <0.5 mm) and three-year-old typical slope protection plants, Pyracantha fortuneana and Amorpha fruticosa, as the research objects, the direct shear friction test of root soil interface was carried out in laboratory. The results showed that: (1) The friction strengths of root-soil and soil-soil interface were positively related to the vertical load. With the increasing of soil particle size, the friction strengths, friction coefficients, cohesions and friction angles of root-soil and soil-soil interface all decreased, and within the same particle size range, the friction characteristics of P. fortuneana root soil interface were significantly better than those of A. fruticosa and plain soil. (2) The shear stress of the root soil interface and the plain soil interface increased first and then decreased with the increasing of displacement. The relationship curve of root soil interface shear stress and displacement showed a zigzag rise, while the plain soil curve showed a gentle rise. (3) The smaller the soil particle size was, the larger the specific surface area of the indirect contact surface between the root and soil was. The small soil particles inlayed with the root surface, which increased the bite force of the contact surface and increased the friction capacity. The above research had a certain theoretical significance for further exploring the mechanical mechanism of root soil interaction of shrub plants, and improving the stability of slope soil in the experimental area by use shrub plants and preventing the shallow sliding of slope.