Abstract: Shallow fine roots enhance soil shear strength and play a critical role in vegetated slope stability. While prior research focused on factors like soil type, moisture, root arrangement, and plant species, the relationship between root characteristics and root-soil composite strength remains underexplored. This study employs direct shear tests on saturated Amorpha fruticosa root-loess composites to systematically investigate the influence of root content, root length, and key parameters (root area index - RAI, root length density - RLD, root volume ratio - R_v) on shear strength. Results show: ①under low vertical loads, shear strength increases significantly with root content, but this effect weakens under higher loads; ② reinforcement primarily enhances cohesion, with minimal impact on internal friction angle; ③ for root length ≥2 cm, an optimal root content (1%-1.5%) maximizes cohesion; ④ shear strength correlates strongly positively with RAI and R_v, but weakly with root content, biomass, RLD, and specific root length, indicating dominance of root-soil interface friction under saturation. This work clarifies the mechanical mechanisms of root reinforcement in saturated loess, identifies a specific optimal root content, defines key root characteristic-strength relationships (RAI, R_v, average diameter), and provides a new theoretical basis for the comprehensive understanding of the mechanical effects of vegetation slope protection in loess areas and the scientific design of ecological slopes.