Abstract: Plant roots can significantly improve the strength of soil. In order to explore the enhancement effect of alfalfa root on the shear strength of loess, study was conducted based on the root-soil composites of alfalfa with a growth period of 60 d herein. Specifically, the parameters on shear strength of the alfalfa root-soil composite and the tensile strength of alfalfa root were obtained through the direct shear tests and root tensile tests. By observing the structure and characteristics of alfalfa root with an electronic magnifier, a modified Coulomb model of root-soil composite was established with the empirical formula. Meanwhile, the advantages and disadvantages of the modified Coulomb model, the WWM model and the modified MMW model were comparatively analyzed. In addition, the plant root enhanced soil strength in the loess area was evaluated. The results show that the alfalfa root system has a significant enhancement effect on the shear resistance of soil. The alfalfa root system is well developed with the root diameter between 0.2-6.7 mm. With the increase of root diameter, the ultimate tensile resistance of a single root increases exponentially, but the tensile strength decreases in the same way. The shear strength of root-soil composite under different Root Area Ratio (RAR, which refers to the ratio of total cross-sectional area of root system to the cross-sectional area of soil) is significantly improved compared with the soil sample without roots, indicating that RAR is obviously correlated with the cohesion and internal friction angle. Definitely, RAR is in linear positive relation to the cohesion and in relation of Gaussian function to the internal friction angle. The modified Coulomb model of root-soil composite shows good reasonability, effectiveness and applicability to the increment prediction of tensile strength of root-soil composite of herbaceous plants in loess area, followed by the modified WWM model, and the WWM model has relatively poor accuracy of prediction. However, in predicting the shear strength increment of woody root-soil composites, the modified Coulomb model and the modified WWM model are much more accurate than the WWM model. The research results are of guiding significance to the vegetation slope protection projects in loess areas.