[Objective] Analyzing the effects of two common plants' root system on improving soil shear resistance quantitively in order to provide some theoretical bases for the evaluation of soil enhancement effect of plant's root system. [Methods] Direct shear and two kinds of model(Wu model, the fiber bundle model) were conducted to compare the shear strengths of root-soil composites with different RAR(root area ratio) of Pinus tabulaeformis and Acer truncatum with that of plain soil. A universal testing machine was used to measure the root's pull-out strength and tensile strength. [Results] (1) Mainly through enhancing soil cohesion, roots could enhance soil shear strength; (2) The relationship between root strength(include tensile strength and pull-out strength) and root diameter agreed with power funnnction. When root diameter D > 2 mm, pull-out strength was less than tensile strength, and vice versa; (3) The Wu model overestimated the shear strength of root-soil composite at an average of 26.81%, and the fiber bundle model also overestimated with an average of 9.82%. [Conclusion] As comparing to the Wu model, the fiber bundle model is more accurate in describing the effect of root's soil enhancement.