To clarify the influence of alfalfa roots on the shear strength of Yili loess, tensile tests on alfalfa roots and direct shear tests on root-soil composites with different root diameters and root-soil cross-sectional ratios were carried out. Meanwhile, numerical simulations of direct shear tests were conducted to determine the optimal root-soil cross-sectional ratio under different root diameters and the variation law of plastic zones. The results show that the tensile force of alfalfa roots increases linearly with the increase of root diameter, the tensile strength of roots decreases following a power function with the increase of root diameter. The shear behavior of alfalfa root-soil composites exhibits strain hardening characteristics. With the increase of root-soil cross-sectional ratio, the range of plastic deformation expands and the peak shear strength increases. Under the same root-soil cross-sectional ratio, the shear strength decreases with the increase of root diameter. For the same root diameter, the cohesion of the root-soil composite increases with the increase of root-soil cross-sectional ratio. Under the same root-soil cross-sectional ratio, the cohesion of the root-soil composite decreases as the root diameter increases. The internal friction angle of the root-soil composite is generally similar to that of plain soil. The variation curve of cohesion versus root-soil cross-sectional ratio follows a quadratic function relationship. With the increase of root diameter, the curve transforms from “convex” to “concave”. For roots with diameters of 0~1 mm, the optimal root-soil cross-sectional ratio is approximately 1.5%. For 1~2 mm diameters, it is slightly larger than 1.5%. For 2~3 mm diameters, it exceeds 2.7%. Under the same root-soil cross-sectional ratio, reducing the root diameter can effectively expand the plastic zone, thereby increasing the shear strength.