[Objective] The effects of vegetation root content on the shear strength of mountain red earth acquired from vegetated slope in Chenggong district, Kunming City of Yunnan Province were analyzed in order to provide theoretical basis for relevant research.[Methods] The shear strength tests were carried out by preparing roots-soil complex samples with four levels of root content:0 (no root samples), 0.467, 0.93, 1.4 mg/cm³, as well as five levels of soil moisture content (20%, 25%, 30%, 35%, and 40%).[Results] The vegetation roots were found to provide soil with additional shear strength and helps soil resist deformation, which can enhance its shear strength. Vegetation roots had obvious enhancement effect on the shear strength of soil, but this enhancement effect was limited. When the root content was 0.93 mg/cm³, the root system had the best enhancement effect on the shear strength of soil. With an increase in root content, the cohesion and the angle of internal friction always presented a negative correlation. In the case of low water content, the cohesion first decreased and then increased, while the angle of internal friction changed inversely with the change of root density. When the soil moisture constant and the root content was low (water content and root content less than or equal to 35% and 0.93 mg/cm³ respectively), the soil participated in the whole process of shear and they were the main object to be sheared, and the calculated cohesion and internal friction angle were both positive, which was consistent with Coulomb's law. When the soil moisture and root content was high(water content and root content was 35% and 1.4 mg/cm³ respectively or the root content was 45% and root content was greater than or equal to 0.467 mg/cm³), the shear object was mainly the root system, and the cohesion calculated by Coulomb's law was negative, which does not conform to Coulomb's law.[Conclusion] Coulomb's law is not applicable to the study of the shear strength of the root-soil complex which has high water content and vegetation density. When using vegetation for slope protection, it is necessary to carry out experiments to determine the optimal root and water content of the slope soil to produce the highest shear strength.