The tooth root bending stress calculation method of high contact ratio spur gear pair was studied. The gear tooth deformation and stiffness calculation of high contact ratio gear and load of single tooth were studied. The definition of load sharing ratio and the calculation method of high contact ratio gear tooth were given. The meshing point of high contact ratio gear pair with double teeth was taken as the point of computational load.The calculation method of tooth shape coefficient and stress concentration coefficient of double toothed meshing zone boundary of the dangerous section that located on position of high contact ratio gear tooth roots 30 degree transition curve were given. Calculation formulas of tooth root bending stress of the dangerous section was obtained. External gear pair with different contact ratios were designed

and the tooth root bending stress was tested by CL100 gear test machine. The test result show that

under high load

the tooth root bending stress theoretical calculation error of active gear is less than 785%

and the driven gear is less than 98%. Under low load

the tooth root bending stress theoretical calculation error of active gear is less than 241%

and the driven gear is less than 19%.