Abstract: In order to explain the phenomenon that the square concrete-filled thin-walled steel tube (CFTST) stub column under axial loading can also reach higher ultimate load capability after its outside steel tube has been buckled, the formula to research the postbuckling mechanical capability of clamed steel plates are deduced by using the small and large deflection theories, the active confined force inside concrete caused by the outside post-local buckling steel plate is then obtained, thusly the ultimate compressive stress of concrete inside CFTST stub columns can figured out by the confined concrete model, the reason that CFTST stub column under axial loading can reach higher ultimate load capability after its outside steel tube has been buckled is explained, it is very useful for the further theoretical analysis.