Plant roots significantly enhance the stability of shallow slopes. However, the mechanism of the root-soil interaction at root-bundle scale remains unclear. The objective of this study is to reveal the factors that control root-soil interface frictional stress at root-bundle scale. Through theoretical analysis, it is clarified that spatiotemporal inhomogeneity exists in root-soil interface frictional stress, and it could be amplified by root density and root deflection angle. Large-scale shear tests with 160 and 10 mm shear zones were then conducted to verify this hypothesis, where variations in shear zone thicknesses corresponded to different shear deflections. The number of roots was measured within the sheared zone. Results reveal that the average root- soil interface frictional stress of rooted soils with 10 mm shear zone is 66% higher than that for rooted soils with 160 mm shear zone. During the shear process, the state of roots transitions from stretching to sliding, accompanied by increasing root-soil interface friction. This study confirms that root-soil interface frictional stress is jointly amplified by root density and root deflection angle, providing robust evidence for constructing physics-based mechanical reinforcement models.