Compared with the conventional gear, the asymmetric gear has better root bending stress and tooth contact stress. Taking a pair of asymmetric involute spur gears as the research object, a model of asymmetric gear considering the influence of friction and shear stress on root bending stress is established. Take the upper boundary point of single tooth meshing area as example, the formula for calculating root bending stress of asymmetric gear is deduced under friction, get the rule of root bending stress without neglecting the friction force and the shear stress by MATLAB. At the same time, we design the plane finite element model of conventional gear (20°/20°) and asymmetric gear (20°/35°) by using APDL. The upper and lower boundary points of the double tooth meshing area, the upper and lower boundary points of the single tooth meshing area, the meshing node are systematically studied, get the change rules of root bending stress in the meshing process under the condition that the friction force cannot be ignored.