Abstract: Contraposed to the lack of testing and evaluation standards of high-pressure compressed natural gas (CNG) vessels, using fracture mechanics approach and probability analysis method, H₂S stress corrosion cracking tests and corrosion fatigue tests were done in the H₂S solution of moderate concentration and at a low frequency 6.7μHz. The relations between fatigue crack growth rate dα/dN and the increment of fracture toughness △K were determined. By using inverse inferring method, considering critical crack sizes of H₂S stress corrosion cracking, crack increments of corrosion fatigue, random capability and reliability of data and the practicality of different service period, the allowable crack sizes of different service period were calculated and the curves between allowable crack depth and original crack length were obtained, which would supply more accurate data to determine the service period and to evaluate pressure vessels' residual life.