Abstract: The residual bearing mechanism of reinforced concrete (RC) columns and the practical calculation method for the residual strength of square columns after exposure to ISO834 standard fire are investigated. Combining the constitutive model of concrete and its reinforcements after exposure to fire and the conjugate method, the numerical model for the calculation of residual strength of RC columns after exposure to fire is proposed and the corresponding program CAFIRE is developed. The second-order effect of the axial force is considered in the study. Using the numerical program CAFIRE, the influence of several correlative parameters on the residual strength of RC columns with square cross section after exposure to ISO834 standard fire is analyzed. 2700 RC columns with square cross section are selected for residual strength analyses. Different fire exposure times, effective lengths, sectional dimensions, load eccentricity ratios, reinforcement ratios and load angles are considered in the analyses. Based on the simulation results, a practical calculation method is proposed for the residual strength of RC square columns. Moreover, the residual strength of RC columns with the same sectional area and reinforcement ratio but with different shape of cross section is compared using the program.