Abstract: Soil erosion and land degradation frequently occur on Loess Plateau in western China. Since the vegetation can effectively reduce shallow landslides, root reinforcement can be an environmentally beneficial soil stabilization measure in recent years. The Loess Plateau also presents a large number of plantation forests with a uniform arrangement. Among them, a periodic distribution can be found in the root system and biological characteristics. The traditional model of root reinforcement cannot consider the effect of roots on the elastic modulus of the root-soil composite. Meanwhile, it is difficult to calculate the shear strength of soil containing a large number of roots for the deformation of the root-soil composite. There is a strong coupling relationship between the plant roots and soil. In this study, a three-dimensional constitutive relation was constructed for the root-soil composite using the homogenization theory, in order to quantitatively the strengthening effect of plant roots on the soil and deformation development under the loads. The root-soil composite was regarded as the natural fiber-reinforced composite material. The equivalent elastic modulus and strength parameters of the root-soil composite Unit Cell (UC) were calculated via the periodic boundary conditions using the finite element software ABAQUS scripting interface. The UC model was added with the six types of load conditions, and then the equivalent stiffness matrix was solved to obtain the equivalent elastic parameters. A triaxial test and numerical simulation were also carried out to verify the strength parameters and calculation accuracy of the root-soil composite UC. Specifically, the UC model was loaded with vertical axial compression and horizontal confining pressure (100, 200, 300, and 400 kPa). A Duncan-Zhang E-B model was selected for the soil, following the Mohr-Coulomb yield criterion. Finally, the micro-mechanical analysis was also performed on the UC. The results show that: 1) The UC model of the root-soil composite effectively predicted the equivalent elastic parameters, where the relative error of axial elastic modulus was 2.2% (1.01 MPa), compared with the test. 2) The axial elastic modulus of the root-soil composite increased from 36.2 to 45.8 MPa under 100 kPa confining pressure, with an increasing ratio of 26%. The root system interacted with the soil, leading to the formation of a stiffer structure in the root-soil composite, compared with the pure soil. There was an improved shear strength of the soil, whereas, the improvement of the soil elastic modulus cannot be ignored in the root system. 3) The UC model can be expected to predict the cohesion c and internal friction angle φ of root-soil composite, where the difference between c and φ was only 4.41 kPa and 0.93°, respectively, compared with the test. The calculation efficiency was greatly improved with the high accuracy, where the element numbers of UC were reduced by 90%, compared with the standard size model. 4) The root system allowed the soil to produce much more deformation and stress. Once the confining pressure was smaller (100 kPa), the peak stress of the root-soil composite was improved up to 13.64% compared with the pure soil. The peak stress of the root-soil composite rose by 4.99%, as the confining pressure was larger (400 kPa), compared with the pure soil. These findings can also provide a new idea for the root reinforcement mechanism on Loess Plateau in China, particularly from the perspective of periodic composites.