Due to the nonuniform constraints of the steel tubes on concrete, the calculations of the axial compressive load-carrying capacity of square concrete-filled steel tubular (SCFST) columns and rectangular concrete-filled steel tubular (RCFST) columns are complex problems. In this work, the calculation of axial compression load-carrying capacity of RCFST columns was finely divided into five parts creatively by applying superposition principle. It analyzed the constraint mechanism of rectangular steel tubes on the concrete, the stress state of the concrete in the strong and weak constraint zones, and steel tubes on the height and breadth sides. A unified model of axial compression load-carrying capacity of SCFSTs and RCFSTs was established. Based on the established database including 449 published RCFST and SCFST columns, the rationality, accuracy and wide applicability of the proposed model was verified. The parameter sensitivity analysis of the factors affecting the load-carrying capacity of RCFSTs was carried out, the influences of parameters on the improvement of concrete load-carrying capacity and the reduction of the axial load-carrying capacity of the steel tube were studied in detail.