Abstract: The slip stiffness of concrete-filled steel tubes is proposed. The axial force, interface shear stress and slip formulas of concrete-filled steel tubular columns under uniformly distributed and concentrated loads are obtained. It is found that the axial compression stiffness of concrete-filled steel tube remains unchanged in consideration of slip. The results of the bond-slip tests of 39 concrete-filled square steel tube specimens and 18 concrete-filled circular steel tube specimens are analyzed and the values of interface slip stiffness are obtained. Concrete-filled steel tubular column models are established in ANSYS. The influence of the thickness of the cap on the stress distribution of concrete and axial compression stiffness of column is analyzed. The analysis results show that the cap on top is important to guarantee the composite action between the steel section and the concrete core.