Abstract: With the development of bioengineering stabilization techniques for slope protection, the role of root reinforcement in slope protection has been paid more and more attention to. The primary root and the lateral root are complementary to each other in most root systems. Therefore, the shear tests with single root often fail to reflect the mechanism of root-soil reinforcement in the real vegetated slopes. In order to reveal the reinforcement mechanism of composite roots and quantify the influences of root angle to the strength of slope soils, the large-scale direct shear tests and numerical simulations are carried out to study the effects of composite roots on the shear strength of soils, with lateral root angles of 30°, 45° and 60°. The results show that composite roots can obviously improve the shear strength of soils, and the maximum shear strength increased by the roots is 6.36 kPa. The external shear load is primarily carried by the lateral root opposite to the shear direction (i.e., the front lateral root), and the consequence of the contribution of composite roots to the shear strength of soil in values is as follows: the front lateral root, the primary root and the posterior lateral root. With the increase of lateral root angle, the axial force and the tensile deformation on the front lateral root increases, and the distribution of the plastic zone near the front lateral root increases, which shows that the front lateral root can mobilize more soils around it to resist shear force, and the soil reinforcement effects of composite roots are enhanced.