According to the strain energy release rate of fracture mechanics, the fatigue fracture parameters were determined, and the calculating model of the fatigue life of O-ring was established by fitting and classifying the specific experimental data obtained. Based on the modal of the fatigue life, the fatigue life of O-ring on dangerous cross section units was calculated. And the same time, the finite element analysis was carried out on the O-ring, and the dangerous section found out, and the fatigue life of nodes on the dangerous section was calculated. The results show that the calculation result of fracture mechanics, the finite element analysis result and the experimental result are basically the same, which proves the rationality to calculate the fatigue life of O-ring by fracture mechanics.