By introducing the symmetrical hypothesis of spherical shell deformation under pressure, the theoretical expression of pressured spherical shell was simplified to plate model. By using a parameter, a uniform expression was obtained with the modulus factor curve and other multimodulus theories. Based on the stressstrain curve, the modulus factor curve and its analytical expression were obtained. Take the tangent modulus theory and double modulus theory as examples,  the two theories were applied to calculate the pressure capacity of Tialloy spherical hull. The calculated values were then compared with the experimental datum. Finally,  the validity and applying scope of the ultimate strength for pressure hull calculation were obtained, providing a scientific reference for the early design of deep sea submarine.