Pitting corrosion can cause the ultimate strength degradation of hull structure. This research aims at the development of ultimate strength calculation formula of pitted stiffened plates under unaxial compression. Firstly, the relative parameters (such as sizes of stiffened plates, sizes and shape of pits, initial geometric imperfection and residual stresses of stiffened plates, boundary condition and type and size of finite element) of pitted stiffened plates in the finite element analysis were analyzed and assumed. Secondly, the effects of a few factors (such as distribution status, depth and position of pits, number of longitudinal stiffeners and sizes of stiffened plates) on the ultimate strength with respect to the corroded volume loss were discussed. Lastly, the ultimate strength reduction formulae of pitted stiffened plates based on corroded volume loss were obtained by the data analysis from lots of nonlinear finite element analyses.