Abstract: The effects of fixing soil by plant roots vary when roots are damaged; however, the mechanical process remains unclear. In this study, Symplocos setchuensis in Jinyun Mountain of Sichuan Province was used as research object. Using 2 mm as root diameter class, we gradually cut off roots in each class to simulate the process of roots being gradually damaged. Simultaneously, the root tensile and pull-out strength were measured by an electronic pulling force machine, and the shear strength of the root-soil composite (RSC) was measured with large direct shear test. The measured results were compared with calculated values of the model developed by Wu (1976), so that the effect of soil fixation of roots in the process of gradual damage to them was systematically explored. Results showed that the tensile strength of 3-year-old S. setchuensis seedlings growing in spring decreased with the decrease of root diameter. Tensile strength works mainly in roots with diameter less than 2 mm. Pull-out strength is the major power in roots with diameter larger than 2 mm. Results from large direct shear test indicated that the shear strength of RSC was consistent in the initial stage of root damage, and it declined rapidly when the roots with diameter larger than 2 mm were separated from the soil. Moreover, the larger the RAR (root area ratio) is, the greater the shear strength of RSC is, and the stronger the effect of soil fixation is. However, to a certain degree,Wu’s model overestimates such effect and the overestimated parts have a positive correlation with RAR.