Abstract: Different types of crack defects may occur in the production and manufacturing process of ultra-high pressure vessel heads. Under the action of life expectancy pressure load, different types and distribution of crack defects have different tolerance limits, which have different effects on the service life and safety of heads. Therefore, Abaqus and Franc3D finite element analysis software were combined to study the allowable limits of surface cracks and deep-buried cracks under the expected life of ultra-high pressure vessel heads, and a crack defect zoning evaluation model was proposed. The results show that the length variation range and the critical size of crack in the stress concentration zone are smaller than those in the non-stress concentration zone. The defect tolerance limits of different types and distribution areas of cracks under life expectancy are determined. The internal relationship between the performance of ultra-high pressure vessel head and the defect tolerance limits is analyzed. The research results provide an important theoretical basis and method for the detection and safety evaluation of the head defects of ultra-high pressure vessels.