Abstract: Using the method of continuum mechanics, small-deformation elastic analysis of concrete-filled steel tube (CFST) is conducted. The elastic constitutive equations, the composite elastic modulus and the composite axial compression stiffness of CFST are derived. It is showed that the composite axial compression stiffness is 6.9%~26.4% larger than the simple summation of the stiffnesses of concrete and steel. The calculated results by the proposed formulas are close to those by unified theory. The theory formulas can accurately and reliably describe the elastic behaviors of CFST at infinitesimal deformation.