This study used the roots of ficus microphylla and heptapleurum heptaphyllum to conduct the pull-out tests for the root–soil composite, focusing on an analysis of the influence of skin morphology and basic tensile mechanical properties of roots on the root–soil interaction. The results showed that the curves of pull-out force vs. displacement for both two types of plant roots display three characteristic stages. The maximum friction force at the root–soil interface increases with the increase of the root diameter and normal load, and the interface friction force between roots and soils is relatively high for the root with more rough bark. With the promotion of extrusion force of soil particles acting on the root bark, the bark roughness and root’s elastic modulus jointly affect the maximum friction mobilized at the root–soil interface. The findings in this study can provide a support for the practice of ecological slope protection.