Abstract: Considered the residual stress as equivalent residual stress load, and the equivalent load is emended by nonlinear FEM. Applied the thickness discounted quantity of plate as perturbation parameter, researched on the large deflection equations of plates with initial imperfections, the thickness discounted quantity of perfect plates and the ultimate capacity equation of plates are acquired by assimilated between the applied plate with initial imperfections and the perfect plate. According to the results of nonlinear FEA and experiments, the discounted-thickness method is practical with good precision. The method suggested in this paper is universal to analyze the ultimate capacity of steel plated structures, and it is a supplement for nonlinear FEA.