Abstract: In large-scale building fires, due to the rising of hot air, the temperature distribution is usually non-uniform in the longitudinal space around a steel column, and the temperature in its upper zone is much higher than that in its lower zone. This phenomenon is very significant after the flashover condition. It is practical to consider the longitudinally non-uniform temperature for the fire resistance design of steel columns. Besides, advantage can be made of this in a performance-based approach to ascertain the stability of a steel column subjected to prescribed fire size. This paper takes the steel column that has no ending restraint as the research object and assumes that the temperature distribution on the cross-section is uniform. The elastic buckling load of the steel column under the field model fire is derived upon the energy method. Under the condition of the simplified two-zone fire model, the elastic buckling load is derived by the equilibrium differential equation and by the energy method, respectively. The derived results are verified by numerical analysis. It is found that: the error of the exact solution derived upon the equilibrium differential equation is less than 3% compared with the numerical simulation results, while the results derived based on the energy method are significantly larger in some cases. In addition, the results show that: if the longitudinally non-uniform temperature distribution is ignored, the elastic critical load will be seriously underestimated, even reaching about 37%, for a steel column with the same length as the unheated length. Considering the influence of the axial restraint, the calculation method of the buckling load of the steel column with longitudinally non-uniform temperature is proposed.