Abstract: In engineering and academic researches, it is always important and difficult to accurately predict the fatigue life and completely describe the fatigue crack growth process of pressure equipment for oil & gas storage and transportation with initial defects under cyclic loading. Taking the 34CrMo4 steel cylinder with initial crack defects as an example, the extended finite element technology is used to analyze the mechanical fatigue response of the cylinder structure under the cyclic internal pressure. The impacts of various parameters (size, direction and location of initial cracks) on fatigue life and crack growth process are also investigated. Finally, the finite element calculation results are compared with the evaluation results of ASME VIII-3, revealing that they are very close. Nevertheless, since the extended finite element technology can more accurately and efficiently simulate the whole dynamic growth process of initial cracks from the beginning of growth to fracture, it is more advantageous to predict the fatigue life and design the damage tolerance of complex structures.