Abstract: In order to study the fatigue fracture failure in the flexsplines of the industrial robot harmonic reducers, the model based on cylindrical shell theory was used to evaluate the circumferential distribution of stress in the flexsplines and the effects of load, wall thickness and diameter on the stress was also analyzed. The result shows that the stress is more sensitive to the wall thickness than the diameter of flexsplines,while the load torque has little effect on the axial and circumferential stress in the flexsplines. Stress of the flexsplines mainly comes from the circumferential bending deformation forced by the wave generator, it also comes from the axial bending deformation and the torsional deformation. Finally,the evaluation method of fatigue failure strength in the flexsplines was proposed based on the study.