The　concrete-filled　double　skin　steel　tubular　(CFDST)　members　have　been　used　in　many　engineering　projects.　The　stiffened　outer　steel　tube　could　be　used　to　further　enhance　the　structural　performance　and　reduce　the　steel　consumption,　while　the　seismic　behaviour　of　the　stiffened　member　needs　investigation.　In　this　study,　tests　of　6　stiffened　CFDST　beam-columns　were　conducted　under　both　constant　axial　load　and　cyclic　lateral　load.　Main　test　parameters　were　the　axial　load　level　and　the　hollow　section　ratio.　The　load,　deformation　and　strain　of　all　specimens　were　recorded　and　analysed.　The　influence　of　parameters　was　discussed　in　terms　of　the　lateral　resistance,　the　ultimate　displacement,　the　ductility　and　the　energy　dissipation　ability.　It　showed　that　the　stiffened　specimens　exhibited　good　energy　dissipation　capacity　under　the　cyclic　loading.　A　finite　element　(FE)　model　was　developed　to　carry　out　the　mechanism　and　parametric　analysis.　The　longitudinal　stiffeners　could　effectively　reduce　the　local　buckling　of　the　tube　wall.　The　ductility　and　energy　dissipation　capacity　of　stiffened　CFDST　members　are　generally　higher　than　that　of　unstiffened　ones.　Finally,　equations　for　predictions　of　the　flexural　stiffness　and　strength,　as　well　as　the　hysteretic　model　of　the　load-deformation　relationship　were　proposed　for　stiffened　CFDST　members.　(C)　2020　Elsevier　Ltd.　All　rights　reserved.