Abstract: With consideration of the double confinement to the concrete from the twin tubes, the stresses in the concrete and the steel tubes were analyzed, and the ultimate compressive strength was calculated based on the twin shear unified strength theory. The strength reduction factor (βf) was deduced according to plasticity theory, and the confining force between the steel and the concrete was also obtained. Then, the ultimate load capacities of the concrete filled twin steel tubes under axial compression was computed. In comparison with the corresponding experimental results, it is shown that the twin shear unified strength theory can be used in the calculation of the axial capacity. The axial compressive strength of composite column was shown to be influenced by the diameter-to-thickness ratio and the diameter of the inner circular steel tube. The optimal design of the inner steel tubes is presented for the engineering application of these composite columns.