In order to study and compare the mechanical properties of the single root and root-soil composite of Populus davidiana, Picea crassifolia, and Pinus tabuliformis, the single root tensile test, the triaxial compression test, and the saturated permeability test were used to get the mechanical properties and the soil-water characteristic curves of the three arbors. The results showed that there were significant differences in the mechanical properties of the single root of the three arbors, and the diameter had a significant effect on the mechanical properties of the single root. The maximum tensile force of the single root was correlated to the power function of diameter and increased with the increasing of diameter, while the tensile strength and the Young's modulus of single root were correlated with the logarithmic function of diameter and decreased with the increasing of diameter. The average of maximum tensile force of the single root of P. davidiana was 0.38 kN > P. crassifolia (0.25 kN) > P. tabuliformis (0.17 kN), the average of tensile strength of P. davidiana (33.39 MPa) > P. crassifolia (27.79 MPa) > P. tabuliformis (18.45 MPa), the average of Young's modulus of P. davidiana (0.17 GPa) > P. crassifolia (0.12 GPa) > P. tabuliformis (0.11 GPa). There were significant differences in the cohesion and the elastic modulus of the three arbors root-soil composite, but there was no significant difference in the friction angle (the mean value was 30.48°). The cohesion of root-soil composite of P. davidiana (10.65 kPa) > P. crassifolia (8.97 kPa) > P. tabuliformis (7.13 kPa), the elastic modulus of P. davidiana (2.17 MPa) < P. crassifolia (2.52 MPa) < P. tabuliformis (4.21 MPa). The soil-water characteristic curves (SWCC) and the permeability function curves of the root-soil composite of the three arbors were significantly different. The permeability of root-soil composite of P. davidiana was higher than that of P. tabulaeformis and P. crassifolia. The result is an important reference for evaluating the ability of the three arbors to stabilize the slope and establishing the numerical model of the slope stability analysis for Helan Mountain.