The simple-shear condition is closer to reality than the direct-shear condition for simulating the mechanical behavior of vegetated soil slope under shallow failure. However, study on simple-shear characteristics for vegetated slope is still insufficient, and there lacks intuitive comparison of characteristics between these two shear conditions. In this study, large-scale simple-shear and direct-shear experiments were conducted on soil permeated by roots of Amorpha fruticosa to investigate the shear strength and stiffness. The stress-displacement relationship of each sample was obtained and further normalized to unify the influence of root content. The results reveal that the direct-shear condition overestimates the shear strength of root-permeated soils (by 41%) and thus the estimation of slope stability based on the parameters of direct-shear condition is not conservative. Furthermore, the initial stiffness of root-permeated soil under simple-shear condition is 34% lower than that under direct-shear condition. The higher strength and stiffness under direct-shear condition are caused by the following reasons: the shear plane does not have the lowest strength, the shear area is decreasing, and the shear zone is thinner. The significant deformation (lower stiffness) revealed by the simple-shear condition facilitates the application of early warning for vegetated shallow landslides.