Abstract: Reinforcing fire-damaged square steel tubular columns with infilled concrete results in composite members that are distinct both from traditional concrete-filled steel tubular columns at room temperature and from concrete-filled steel tubular columns after exposure to high temperatures. The axial compression performance of such columns was investigated through experimental and numerical analysis methods. Considering the effects of temperature and of initial stress from preloading prior to reinforcements, axial compression tests were conducted on a total of 10 specimens to study the variation laws of their failure modes, of their bearing capacity, of their stiffness and, of their ductility. A finite element analysis model calibrated by the test results was also established. The influences of temperature, of initial stress, of slenderness ratio, of steel strength and, of concrete compressive strength on the bearing capacity of the reinforced steel tubular columns were studied. A calculation formula for the ultimate bearing capacity of fire damaged square steel tube columns strengthened with concrete filled in was proposed, along with construction suggestions and key precautions for such reinforcements. The research results indicate that: temperature and initial stress have no significant impact on the failure mode of the reinforced steel tubular columns; reinforcing with infilled concrete can significantly improve the bearing capacity, the initial stiffness, and the ductility of fire-damaged steel tubular columns; the temperature, the slenderness ratio, the steel strength, and the concrete compressive strength are the main factors affecting the bearing capacity of the reinforced specimens. Specifically, the bearing capacity has a linear negative correlation with temperature and the slenderness ratio, while showing a linear positive correlation with steel strength and concrete compressive strength. All these factors act independently without significant interactive effects; the errors between the results calculated by the formula proposed and those obtained from tests and numerical simulations are within 10%, which can provide a reference for the reinforcement and for the repair of fire-damaged steel tubular columns using the infilled concrete method.