Abstract: A new design model for column subjected to heavy loads is presented in this paper. This kind of column is formed by inserting steel section into concrete-filled steel tube, and is called circular steel tubular column filled with steel-reinforced concrete. A total of 13 axially loaded composite columns with various combinations of confinement index, structural steel index and slenderness ratio were tested. The mechanical behavior of the composite column is studied. Results show that the load-bearing capacity and ductility of the composite column are improved, and the propagation of cracks in concrete is mitigated or arrested effectively as concrete and steel work together. With the increase of slenderness ratio, the load-bearing capacity and ductility of the composite column decrease. Based on the test results, a numerical model for predicting the load-bearing capacity of the composite columns under axial load is developed. The calculated results are in good agreement with the test data.