Due to the loose and non-cohesive nature of sandy soil, road subgrade slopes in sandy regions are highly unstable, often experiencing adverse geological disasters such as shallow landslides and uneven settlement. To mitigate these hazards and promote sustainable development, vegetation root systems are frequently employed for slope stabilization. This paper investigates the reinforcing efficiency of roots in sand matrix via single-root tensile tests and mixed-level orthogonal direct shear tests. The tensile tests reveal that the peak tensile force (F) exhibits a robust linear increase with root diameter (d) (F = 26.81d − 11.33, R² = 0.80), whereas the ultimate tensile strength (RM) decays non-linearly via an inverse power function. Orthogonal direct shear tests demonstrate that root orientation is the governing factor controlling both composite shear strength and apparent cohesion, exhibiting a hierarchical influence priority of Orientation > Quantity > Diameter. Specifically, root inclusions primarily enhance the ultimate shear strength by increasing the cohesion, with negligible variation in the internal friction angle. The optimal root configuration for maximizing overall shear strength adopts 1.5 mm-diameter roots, a root number of 6, and an inclined orientation. Mechanistically, shear strength enhancement of the composite arises from a cooperative effect of interface friction, axial tensioning/anchoring, and structural interlocking, forming an anisotropic, parameter-specific system rather than a uniform strength upgrade. Thicker roots, under inclined or vertical arrangements, actively mobilize their tensile capacity (F) to resist shearing force. Conversely, three-dimensional vertical-horizontal (cross) networks maximize spatial confinement and structural interlocking, selectively optimizing the apparent cohesion to a peak value of 15.67 kPa. These findings provide a mechanical reference and root-architecture optimization strategies for slope stabilization projects in arid environments.