To investigate root-soil interaction and quantify the enhancement of soil shear strength by herb roots, an improved root-soil reinforcement model based on Wu and Waldron’s model was proposed by integrating three key factors: root distribution shear direction, anchorage length, and effective root number. The enhanced model was employed to examine the impact of the initial angle between the root and the shear surface, as well as root diameter, on soil reinforcement capacity. Results indicate that the improved model accurately fits the experimental data. Specifically, with the increase of the angle between root and shear plane, the reinforcement contribution of inversely oblique roots increases first and then decreases, while that of forward oblique roots increases first from zero and then decreases. Furthermore, with the increase of root diameter, the contribution of roots with varying distribution angles to soil reinforcement increases, roughly exhibiting a proportional relationship.