The ultimate strength formula of short columns under axial compression, which were given by the current code GB 50936-2014 and the current specification CECS 28:2012, were compared and analyzed to improve the theory of ultimate strength of long concrete-filled steel tubular columns under axial compression. The formula of the ultimate strength of short columns, basing on confinement coefficient which was given by GB 50936-2014, was rewritten as a unified form. A new stability coefficient was proposed for long concrete-filled steel tubular columns under axial compression. The accuracy of the proposed stability coefficient was validated by 36 specimens. The results show that the formula of short columns based on the confinement coefficient and the proposed stability coefficient can provide good prediction of ultimate strength for both short and long concrete-filled steel tubular columns under axial compression.