Abstract: To investigate the mechanical performance of square concrete-filled steel tube (CFST) columns reinforced by circular steel tubes at elevated temperatures, a nonlinear finite element (FE) model of the composite column in fire was established. The model is on the basis of modification for the inner concrete stress-strain relationship of CFST columns at elevated temperatures. Comparison between the measured data and predicted results was carried out to verify the FE model. Based on the verified FE model, the fire resistance performance analysis of composite columns reinforced by circular steel tubes with different parameters was conducted, elaborating the rules of axial bearing capacity of the circular CFST section when the composite columns were exposed to fire. Typical deformation curves were analyzed in this paper. Research results show that:1) the inner circular steel tube could remarkably enforce the fire resistance of composite CFST columns; 2) the fire resistance of the composite columns was two times higher than that of normal CFST columns; 3) the fire resistance increased with the diameter-width ratio of D/B and then reduced; however, the fire resistance increased with the coefficient of confinement effect of circular steel tubes. It is concluded that subject to rational design, the bare CFST column reinforced by circular steel tubes are able to meet the national requirements on fire protection.