Abstract:When pressure cabin is diving into a certain depth and diving back, the external water pressure can be simplified as the low-cycle pressure. Combined with specific engineering applications, based on the fracture mechanics theory and the principle of structure damage-safe design, the critical size is used to calculate the device life under the condition of cycle pressure. The device usage time in theory is obtained under the condition of a certain initial crack depth and length. The computer program is used to realize the iterative calculation. This application has effect on the engineering in the fatigue reliability calculation.