This study deals with the elasto-plastic beam-column behavior of circular steel tube columns infilled with reinforced concrete subjected to fire. Based upon the stress-strain characteristics of materials at high temperatures and the mechanics of column deflection curves, a numerical analysis method has been developed to simulate the time historical response. This method can provide a fairly good estimation for the fire resistant performance of the column. It is found that, comparing a proposed simpler superposed strength theory with the above refined numerical solution, the theory gives a satisfactory approximation to predict reinforcement effects of both steel bars and hoops on the ultimate strength of such columns.