Abstract: The ultimate strength analysis of pressure hull is one of the key technologies to design the manned deep-ocean submersible. Based on the tangent modulus theory, introduced the parameter named tangent modulus factor, and combined with the experiment datum of the mechanical properties of materials, the material curves of tangent modulus factor can be built up, which widely applies in the marine engineering and aircraft engineering alloys. Concentrated on the properties of material datum, the simplified formula for calculating the ultimate strength of pressure hull is induced. By using this formula to calculate the ultimate strength of pressure hull that usually uses titanium alloy or TC4 alloy, the results are compared with the FEM data and the experiment data, and it illustrates that this simplified formula has good applicability for engineering. It proves that this method is convenient and effective as a simplified method to analyze the ultimate pressure hull in the designing stage.