Abstract: Q345R steel has good comprehensive mechanical properties and process properties and is one of the most commonly used materials for manufacturing pressure vessels. Cracks are the leading cause of failure and damage to pressure vessels. Therefore, it is of great significance to simulate and analyze the generation and development of cracks during the detection and maintenance of pressure vessels. In this paper, two typical crack forms, center crack and one-sided crack often appear in the process of Q345R steel. The extended finite element method is used to simulate the crack propagation process of a two-dimensional flat plate center crack under tensile state and one-sided crack under low cycle fatigue. The stress changes and strain changes at the crack tip during the crack propagation process are analyzed, and the crack propagation law is also studied. The results show that stress concentration occurs at the crack tip during crack propagation, The stress at the crack surface is the smallest, the stress at both ends of the crack surface is symmetrically distributed, and the whole stress distribution trend is consistent with the theory of fracture mechanics. It is proved that the extended finite element method is used to simulate the reliability of crack propagation.