[Objective] The tensile and flexion mechanical properties and stress-strain characteristics of roots of Pinus massoniana were studied to enrich the mechanical properties of tree roots, in order to provide a reference for soil stabilization and soil erosion prevention in planting areas of Pinus massoniana. [Methods] The 0—1, 1—2, 2—3, 3—4 and >4 mm root diameter roots of Pinus massoniana were taken as the research object. STS5000 universal tensile machine was used to conduct single root test to analyze the tensile and flexion mechanical properties and stress-strain characteristics of different root diameter roots of Pinus massoniana. [Results] Both the tensile strength and flexural strength of a single root of Pinus massoniana increased with increasing root diameter in accordance with the polynomial relationship of y=ax³+bx²+cx+d, having respective R² values of 0.91 and 0.69. Both the tensile strength and flexural strength decreased with the increasing root diameter according to y=ax^-b, having respective R² values of 0.50 and 0.35. The root tensile elongation rate increased with increasing root diameter according to the equation y=ax³+bx²+cx+d (R²=0.17), while the root flexural elongation rate had no significant correlation with increasing root diameter. Both the tensile elastic modulus and flexural elastic modulus of the root system decreased with increasing root diameter according to y=ax³+bx²+cx+d, having respective R² values of 0.22 and 0.18. The root tensile and flexural stress-strain curves of Pinus massoniana showed four stages: elastic deformation, elastic-plastic deformation, strain hardening, and peripheral (fracture) failure. The tensile stress-strain curve generally showed an “upward convex” shape, while the bending stress-strain curve generally showed a “downward concave” shape. The tensile and flexural fracture stages were divided into peripheral fracture and overall fracture modes, both of which were influenced by the peripheral and vascular tissues and had two types of fracture. [Conclusion] The flexural mechanical properties of the root system of Pinus massoniana with a root diameter of <3 mm were superior to the tensile mechanical properties, while the tensile mechanical properties of roots with diameters of >3 mm were superior to the flexural mechanical properties. Planting Pinus massoniana in slope areas can effectively increase soil erosion resistance.