<p>At present, the research on the root-soil composite mainly focuses on the mechanical properties under conventional stress paths, failing to fully reveal its three-dimensional strength and deformation characteristics under complex stress paths. In view of this, using the true triaxial testing system, a constant minor principal stress (<i>σ</i>₃) and constant intermediate principal stress coefficient (<i>b</i>) shear test was conducted on the root system loess to study the influence law of <i>b</i> on the mechanical properties of root system loess under varying confining pressure. The research results show that the generalized stress-strain curve is of the strain hardening type. The intermediate principal stress significantly enhances the hardening effect of the sample; the change of <i>b</i> value mainly causes a significant change in the cohesion of root-loess complex, and the cohesion reaches a peak when <i>b</i> = 0.5; Under the stress condition of <i>b</i> = 0.5, as the content of roots increases, the cohesion of root-loess complex shows a trend of first increasing and then decreasing; there is a critical <i>b</i> value that makes the sample approach the plane strain state, and as the <i>b</i> value increases, the deformation mechanism changes from being dominated by hydrostatic pressure (<i>b</i> &lt; 0.5) to being controlled by the Lode angle (<i>b</i> &gt; 0.5), and the dependence of deformation on the Lode angle first increases and then decreases. This research has practical significance for the soil reinforcement of vegetation restoration in the Loess Plateau by herbaceous roots.</p>