The　nonlinear　finite　element　method　(FEM)　was　used　in　the　paper　by　using　LS-DYNA　software　to　numerically　study　the　failure　mechanism　of　light　steel　columns　under　fire　and　explosion.　The　constitutive　model　under　the　combined　effects　of　fire　and　explosion　was　established,　and　the　numerical　simulation　analysis　of　the　columns　under　combined　effects　of　fire　and　explosion　was　performed.　The　results　showed　that　the　failure　modes　of　the　mentioned　columns　were　all　bending　shear　failures.　The　deformation　diagrams　of　the　column　under　the　blast　and　different　temperatures,　respectively,　were　proposed　and　this　will　provide　a　foundation　for　establishing　the　failure　modes　and　the　simplified　calculation　method.　According　to　the　relationship　between　rotation　angles　at　the　ends　of　the　column　and　temperatures,　the　damage　index　of　D　was　proposed　in　this　paper　for　the　evaluation　of　the　damage　level　of　the　simulated　light　steel　column