Grooving corrosion results in a decrease in the ability of the structure to resist external loads. In the present study, a new assessment method was developed to investigate the ultimate loading capacity of stiffened plates with grooving corrosion damage. First, the basic parameters of stiffened plates (including model range, boundary condition, welding residual stress, initial geometric imperfection, and size of finite element) were assumed. Second, the influences of corrosion parameters and geometrical parameters of stiffened plates (such as finite element type, groove width, groove depth, groove depth-to-width ratio, plate flexibility, stiffener flexibility, and number of stiffeners) were analyzed. Third, based on the data analysis from a large number of nonlinear finite element analyses, the ultimate strength reduction formula of stiffened plates was derived. Last, the correctness of the formula was verified by ultimate strength experiment.