Flexural-torsional buckling of simply supported steel beams in high temperature was studied based on elastoplastic theory.The flexural-torsional buckling critical moment equations were driven.The derivation steps contain:establishing the relationship between bending moment and curvature

making elastoplastic analysis in bending moment plane.An example was provided to analysis the variation of the critical buckling moment with temperature in the elastoplastic stage.ANSYS software was used to simulate the vertical deformation of the steel beam in different burning times.It is known that the critical moment in the elastoplastic stage reduces with the temperature rise.While the vertical deformation of the steel beam increases with the temperature rise.The variation details of the mentioned two variables can also be found in this paper.