Pressure vessels are widely used in petrochemical, aerospace, transportation and other industries. In the process of processing, it will form surface opening defects, and these defects will threaten the production safety. In order to effectively avoid the occurrence of safety accidents and understand the service life of pressure vessels, it is necessary to predict and analyze the crack limit size. In response to the problem of complex stress and difficulty in theoretical calculation after cracks appear on the surface of pressure vessels, this article uses the ANSYS Workbench platform to simulate and analyze the pressure vessels and obtain relevant parameters such as stress intensity factor and stress. Through calculation, the toughness ratio and load ratio are obtained. Using the FAD Failure Assessment Diagram, the ultimate crack size of pressure vessels under safety conditions is obtained. This method provides a reference for crack analysis methods and safe use of pressure vessels.