The　hysteretic　tests　of　three　concrete-filled　square　cold-formed　thin-walled　steel　tubular　(CSCTST)　specimens　with　longitudinal　stiffeners　were　carried　out.　The　main　experimental　parameter　was　axial　compressive　ratio.　The　experimental　results　indicate　that　the　local　buckling　of　the　steel　tube　is　delayed　by　longitudinal　stiffeners.　The　hysteretic　curves　of　composite　columns　are　plump.　This　composite　column　has　a　good　energy　dissipation　capacity.　With　the　increasing　of　the　axial　compressive　ratio,　the　lateral　load-bearing　capacity　of　specimens　increases　slightly,　but　the　ductility　and　energy　dissipation　capacity　decreases　dramatically.　When　the　lateral　displacement　is　beyond　6　times　of　yield　displacement,　the　specimen　with　the　largest　axial　compressive　ratio　achieves　the　highest　speed　of　stiffness　degradation.　The　finite　element　(FE)　model　of　the　CSCTST　specimen　was　developed.　Through　a　comparison,　it　can　be　found　that　FE　results　agree　well　with　test　results.　The　mechanism　analysis　and　parametric　analysis　of　CSCTST　columns　were　conducted　using　the　FE　model.　It　can　be　found　that　the　concrete　strength　can　be　effectively　enhanced　due　to　the　confinement　of　the　square　cold-formed　thin-walled　steel　tube　with　longitudinal　stiffeners.　The　local　buckling　of　the　steel　tube　is　observed　after　the　peak　load.　The　steel　tube　buckling　only　occurrs　between　the　longitudinal　stiffener　and　the　corner　of　the　steel　tube.　The　strength　of　this　composite　specimen　is　significantly　affected　by　parameters　including　the　material　strength,　axial　compressive　ratio,　width　to　thickness　ratio　of　the　steel　tube　and　slenderness　ratio.　The　shape　of　the　load　versus　displacement　skeleton　curve　of　the　composite　column　is　affected　by　parameters　including　the　concrete　strength,　axial　compressive　ratio　and　slenderness　ratio.　Based　on　the　parametric　analysis,　the　simplified　hysteretic　model　was　proposed　for　this　composite　column.　The　predictions　using　the　simplified　hysteretic　model　agreed　well　with　the　FE　results.　©　2019,　Editorial　Office　of　Journal　of　Building　Structures.　All　right　reserved.