Abstract: To investigate the similarity of fire behavior of reinforced concrete columns subjected to fire exposure on four sides, this study conducts fire tests and numerical simulations on six RC columns with different scale ratios and load ratios based on existing similarity theories. The similarities in temperature fields, in deformation development, and in fire resistance limits between the model columns and the prototype columns are compared, and the impact of concrete spalling on their similarity is examined. The study finds that: when the relationship between the heating curves of the model columns and the prototype columns satisfies the thermal radiation compensation model, and neither significant spalling occurs in both types of columns or they are in non-spalled regions, adopting a fire exposure time similarity constant of S² (where S is the ratio of the characteristic geometric dimensions of the model to that of the prototype) can achieve a good similarity in the temperature development curves at corresponding positions between the model and prototype columns. However, due to the nonlinear effects of high temperatures and the size effect of concrete materials, the predicted fire resistance limit of the prototype column using the relationship between the scaled-down model column's fire resistance limit and S² is overestimated; when spalling occurs in concrete columns, the overestimation is even more pronounced because the concrete spalling in large-scale prototype columns is more severe than in small-scale model columns, leading to a greater reduction in the fire resistance limit of the prototype columns. For columns with severe concrete spalling and a higher load ratio, their fire resistance decreased significantly, compared to those without spalling, with the highest decrease reaching 67%.