The purpose of this study concerns with the plastic analysis of cable-stayed bridges by using increment-varied stiffness method. In the increment-varied stiffness method, the nonlinear plastic problem is treated as a piecewise linearized problem. Three kinds of systems, i.e., floating system, multi-supported floating system and pylon rigidly connected to the girder and continued to a lower pier, are used in this research. A finite element computation procedure using increment-varied stiffness method is setup for the nonlinear plastic analysis of those structures to evaluate the ultimate load-carrying capacity. The results will assist the rational retrofit of existed cable-stayed bridges.