[Objective] The mechanical properties of the root-soil system of Yunnan pine and its influence on slope stability in the southwestern mountainous area of China were determined in order to provide a theoretical basis and data support for the understanding, identification, early warning, and prevention of shallow landslides in this area. [Methods] Research regarding the root system of Yunnan pine is extremely lacking as it relates to shallow landslides in Liangshan Prefecture, Sichuan Province. The shear strength test of the root-soil system and FLAC 3D numerical simulation were carried out to explore the mechanical properties of tree-root-soil system and shallow slope stability under different combinations of root density, root distribution, and soil water content. [Results] Higher plant root density could significantly improve soil shear strength. Compared with soil without roots, the cohesion and internal friction angle of soil with roots increased by 45.5% and 9.6%, respectively. However, there was a significant negative power function relationship between root diameter and root tensile strength (p＜0.05), and the tensile strength of fine roots with diameters less than 2 mm was the strongest. At the same time, under the horizontal or vertical distribution of the root system, an increase in soil water content would reduce the shear strength of the soil, and this situation was not conducive to slope stability. Staggered distribution of trees was most conducive to improving soil shear strength and slope stability. [Conclusion] The characteristics of high density, complex staggered distribution, and clustering of fine roots play an important role in increasing the shear strength of the root-soil system and slope stability. Moderately increasing vegetation coverage is one of the important measures that could be implemented to control the occurrence of shallow landslides. Evaluating the effect of soil protection and slope stabilization provided by tree roots under different conditions in geologically disaster-prone areas could provide theoretical support for ecological restoration in geologically active areas after earthquakes.