Four typical plants are chosen to analyze the reinforcing mechanism of plant root and form a simplified calculation method of shear strength for root-permeated soils. A uniform function for the area-depth distribution of plant was proposed based on these four typical plants. The relationship of tensile strength and root diameter, correlation of plant height to sectional area of root trunk at ground surface, and that of sectional area of root trunk at ground surface to maximum vertical depth from the surface were statistically analyzed. Then, reinforcing mechanism of root-permeated soil was explained based on the reinforcement principle. A simple method to calculate the shear strength of root-permeated soil was proposed according to a uniform function for the correlation of reduced shear strength to depth. The shear strength of root-permeated soil can be directly calculated from the tensile strength of roots and the distribution of root area to depth. At last, the feasibility of proposed method was validated by existing data.