Roots can mechanically reinforce soils against landslides, but the impact of their typically random and complex distribution on this reinforcement is not well understood. Here, using a modelling approach based on homogenization theory, we aim to assess the effect of the randomness and complexity of root spatial distribution in soils on the mechanical properties of the soil-root composite and the resulting reinforcement. To do this, we modeled the soil-root composite as a three-dimensional (3D) soil column through which parallel roots penetrate vertically. The unit cell (UC) of the soil-root composites with a nonuniform root distribution was created based on the characteristics of root diameter distributions of Elymus dahuricus measured in the field, and the equivalent elastic modulus and strength parameters of the composites were calculated. The accuracy of the homogenization method was verified by direct shear tests with undisturbed soil-root samples. The results showed that the UC model of the soil-root composites could effectively predict its equivalent elastic parameters. A parametric analysis using the proposed homogenization model showed that roots can mobilize significant soil portions to resist deformation by increasing both the number and complexity of root distributions, even at the same root volume ratio. This makes the stress distribution in the soil more uniform and improves the shear strength of the soil-root composites. The presence of Elymus dahuricus roots significantly improved the shear strength of the soil-root composites, primarily due to an increase in cohesion of 23%. This study presents a new perspective on the development of a constitutive model for soil-root composites and highlights its potential value for engineering applications that use roots to reinforce soils.