Abstract: According to the former experimental researches of the author and students here and abroad on the bond slip between the concrete and steel tube, the distribution of the bond stress along interface length between the concrete and steel tube in CFST was analyzed in detail. For accurately presenting the bond-slip relationship, the mutual relations among stresses of the steel tube, the bond stress and relative slip between the steel tube and concrete are first deduced through theoretical analysis, and then the equations of stress in the steel tube, bond stress, and relative slip are derived mathematically. On the basis of these equations and the pull out test data, the position functions reflecting the variation of the bond-slip relationship at different positions are proposed. The bond stress expression obtained in this study related not only to the relative slip, but also to the different position. The influence of steel ratios to bond stress was analyzed. This constitutive relationship can be contributed to the further analysis of nonlinear behavior of the CFST structures by Finite Element method.