In this paper, the undisturbed roots-soil composites of Iris tectorum Maxim., the dominant species from the source area of Lanniqing landslide, Zhaotong, were taken as the research objects, and the shear characteristics of the composites were studied in depth by direct shear tests, and compared with the experimental results of remolded composites with the same water content in lab. The results showed that the samples with no roots as a whole had the characteristics of strain softening, while the composites had the characteristics of strain hardening. The shear strength of composites was higher than that of samples with no roots, which increased with the increase of roots contents, so that the roots strengthening effect was obvious; however, the undisturbed composites did not exist the optimal root content similar to that of the remolded composites. The c value of composites has no obvious change rule, while the φ value increased gradually, and both increase rates also have the same trend. The sections of failure composites showed that the ability of roots to consolidate soil was optimal when they were the taproots and they were vertical through shear plane, and the vertical pressure was high. The diameter of lateral roots without taproot was smaller, and the inclined distribution in the composites, especially along with shear direction, would affect roots reinforcing effect. The failure mode of samples with no roots was the overall shear failure with incomplete horizontal failure surface, while the failure mode of composites was that the roots were pulled out of sample instead of being sheared or pulled out, and obvious relative displacement occurred between roots and soil. The results could provide a theoretical basis for the selection of slope protection vegetation in order to enhance the shear strength of composites.