Stability is an important issue in steel structure design.When the steel member is subjected to elastic-plastic instability, the axial compression stiffness reduces with the increasing axial pressure and bending moment at end. Therefore, the authors analyze the process of steel member that is instable in elastic-plastic state in this paper by studying the degradation laws of axial compression stiffness. The eigenvalue buckling and nonlinear buckling analyses of axially compressed member and compression-bending member are carried out by using commercial package ANSYS in this study. The relationship curve between the axial force at end and the axial compression stiffness and the relationship surface among the axial pressure at end, bending moment at end and the axial compression stiffness are determined respectively. The made observations indicate that it is feasible to analyze the process of the steel member that is instable in elastic-plastic state by investigating the degradation properties of axial compression stiffness, which becomes lower with axial pressure at end and bending moment at end.