[Objective] The effects of water content, root content, and root arrangement on the shear strength of the root-soil composite were analyzed to reveal the reinforcement mechanism of Amorpha fruticosa root system on loess slopes, in order to provide a reference for the ecological protection of loess slopes and engineering practices for improving slope stability. [Methods] A shear test of the root-soil complex of A. fruticosa was conducted using a ZJ strain-controlled direct shear apparatus. This study analysed the effects of various water contents, root contents, and root distributions on the shear strength, with non-root soil serving as the control. Additionally, the FLAC 3D software was used to simulate the influence of the root distribution of A. fruticosa on the stability of the loess slope. [Results] ① The cohesion and internal friction angle of the shear strength index decreased with increase in water content. ② With an increase in root content, the shear strength exhibited different patterns under different water content levels. When the water content was 9.31%, the shear strength initially increased with an increase in root area ratio (RAR) but then decreased with further increase in RAR. At RAR was 0.15%, the shear strength reached its maximum value. However, when the water content was 15.65% and 17.44%, the shear strength in RAR. ③ The “卄” shape root arrangement can significantly improve the shear strength of the root soil complex. When the water content was 17.44%, the average shear strength of the root-soil composite increased by 31.81 kPa compared to the control. ④ Numerical simulation results indicated that both root length and the angle between the main root and vertical direction were positively correlated with stability. [Conclusion] The findings from laboratory tests and numerical simulations demonstrated that A. fruticosa can enhance the stability of loess slopes and that an interlaced planting pattern of A. fruticosa provides a more effective protective effect.