A new type of composite column, i. e. structural steel and concrete-filled steel tube (SCFT), was proposed. In the composite column, structural steel was embedded in concrete-filled steel tube (CFT). The mechanics of the composite columns were analyzed, including stresses, strains of concrete, steel tube and structural steel, and buckling of steel tube. The results show that the column has higher bearing capacity and excellent ductility. The ultimate bearing capacity and ductility of the column decrease with the increase of slenderness ratio. The strength of concrete depends on extremely the thickness of steel tube and sizes of structural steel. The distribution of strength of concrete varies with position in cross section even at the same load. Based on above analysis, formulas for predicting the strength of concrete and ultimate bearing capacity of the composite columns are suggested.