Abstract: A constitutive relationship for square concrete-filled steel tube with binding bars (square CFT-WB) is put forward based on the analysis of interaction within the components of square CFT-WB and the conception of equivalent lateral compressive stress. The constitutive relationship takes a similar form of the constitutive model of confined concrete while its parameters are modified to fit square CFT-WB according to experimental results. Finally, a calculation of complete load-stress relationship curves is conducted for some certain experimental specimens using this constitutive relationship. The comparison between the results of calculation and the results of experiment shows that they agree well with each other.