Abstract: Abstract: Helan Mountain is an area prone to geological hazard and an ecological shelter in Yinchuan Plain. Plants have been widely used for controlling shallow landslides and soil and water loss in this area. The exact study site is located at the Tu'erkeng forest protection point in Suyukou national forest park. To reveal mechanisms of slope stability reinforced by root-soil composite, we built a finite element numerical model to simulate the impact of root-soil composite on slope stability. Picea crassifolia was selected as research object, which is one of the most important trees for soil and water conservation in Helan Moutain. In this paper, direct shear tests and triaxial tests of root-soil composite and pure soil were conducted for getting soil strength parameters, and based on the soil parameters, the finite element numerical model for the stability calculation of Picea crassifolia forest slope was established. All the soils, including the root-soil composite and the plain soil, were undisturbed samples and were obtained from the same time and field. In the numerical model, the soil was defined as ideal elastoplastic material and following the Mohr-Coulomb yield criterion. The soil and rock were divided by triangular elements of plane strain (CPE6MP and CPE3). Using the model, factor of safety, maximum plastic strain, maximum displacement, failure time, and their growth rate with the slope grade (18.43°, 21.80°, 26.57°, 33.69° and 45.00°) were studied. In this paper we also calculated the grey relational coefficient between the factor of safety and soil strength parameters including friction angle, cohesion, and dilation angle. It was found that: 1) The relationships between factor of safety and slope grade are similar in the slopes with or without trees, and they all decrease in power function. The growth rate of factor of safety increases with the increasing of the slope grade and shows the exponential change. The steeper the slope, the stronger root-soil composite improves the slope stability. 2) The existence of the root-soil composite can prolong the failure time of the slope, and the failure time of the slope with trees is longer than that without trees. The growth rate of the slope failure time increases with the increasing of the slope grade, showing a parabolic curve, and the steeper the slope, the more obvious the effect. With or without plastic-perforation zone, the maximum plastic strain, maximum displacement and failure time of the slope will have an obvious difference. 3) In the 3 soil strength parameters, the cohesion has the greatest influence on the slope stability. The next is the friction angle and the dilation angle. Moreover, this order is not affected by the slope grade. This is also one of the fundamental reasons why vegetation can affect slope stability obviously. The successful implement of this research can play a key role in understanding the nature of root-soil composite reinforcement, promoting numerical simulation study on slope stability and preventing shallow landslides and soil and water losses in Helan Mountain.