Based on the twin shear unified strength theory which considers the influence of intermediate principal stress and the strength-differential of materials，the mechanical behavior of concrete-filled steel tube(CFST) short column is investigated. Whilst considering the influence of slenderness ratio， unified solution of ultimate bearing capacity for CFST column with initial stress under axial compression is developed. Through comparing the results of proposed formula with that of experiments，the rationality of proposed formula is proved. Furthermore，according to the investigation，a new influence coefficient of initial stress is deduced. The results can provide some theoretical references for the study of CFST columns considered the effect of initial stress. And the solution may have some important practical value.