Roots can be seen as natural soil reinforcement material. The prediction and quantitative evaluation of the shear strength of root-permeated soil is the focus of vegetation slope protection, in which the bonding effect of the root-soil interface is the key factor. Taking the roots of Chinese fir trees as an example, the shear resistance test of root-soil interface bonding strength and the direct shear test of root-permeated soil with different root area ratios and inclination angles were carried out. The results indicated that the bonding strength of the root-soil interface could be quantified by interfacial cohesion and friction angle. The shear strength of root-permeated soil increased with the root area ratio, and its relationship with the inclination angle of root relative shear direction was: 45 degrees > 90 degrees. In addition, an estimation model of the ultimate shear strength of root-permeated soil was developed, in which the bonding effect of the root-soil interface was quantified by the interface bonding strength parameters. The soil stress, root diameter, root length, and the initial angle between the root and shear direction can be considered in the estimation model. The rationality and accuracy of the estimated model were verified through the comparison of experimental results and Wu's model. The proposed model can be used to calculate the stability of the biotechnical reinforcement landslides and evaluate the shear strength of the root-permeated soil.