Abstract: The structural characteristics of plant roots, including morphology and topological structure, have significant effects on the mechanical characteristics of root-soil composites. The mechanical behaviors of the root-soil composites at different time are obtained by carrying out the Lolium perenne root system morphology parameter tests and direct shear tests. By using the self-developed MechRoot program, a numerical model for the root-soil composites with more realistic root structural characteristics of Lolium perenne is established, and the axial force levels and proportions of roots with different shapes and topological structures in the process of direct shear are studied, which illustrates the influences of morphology and topological structures of roots on the mechanical characteristics of root-soil composites and the mechanism of soil consolidation. The results show that the plant roots can strengthen soil significantly, and the increase of shear strength of root-soil composites is mainly caused by the increase of cohesion, with the maximum increase of 4.99 kPa. During the shearing process, with the increase of the morphological complexity of roots, the more range of soil can participate in the resistance of shear by the roots, the shear zone and plastic zone around the roots increase and are concentrated around the roots, and the shear strength of the root-soil composites increases. At the same time, the effects of the roots at several levels change gradually. The axial forces of the primary roots, secondary roots and fibrous roots are 3.87, 1.50 and 0.15 N. With the growth of the roots, the proportion of the secondary roots and fibrous roots participating in soil fixation of the roots increases continuously, with the maximum contributions being 43.69% and 13.80%, respectively.