This paper presents a simplified plastic zone method with consideration of the in-fluences of the PDelta. residual stress ,degradation of crosssectional stiffness and plastic zone lengthon the stiffness matrix for second-order elastic-plastic analysis of steel frames. The method not onlymakes up for a insufficiency which is that the degradation of crosssectional stiffness and plasticzone length in a member ean not be considered by a plastic hinge method but also overcomes a defectwhich is very costly and time-consuming because a fine mesh discretization across the crosssectionand along the member length is required by a plastic zone methed. Analysis result Shows proposedsimplified rnethod is simple and practical.