In　order　to　study　the　effect　of　diameter-thickness　ratio　(D/t)　on　seismic　behavior　of　concrete　filled　steel　tubular　(CFST)　columns,　two　specimens　with　diameter　600　mm　were　tested　under　constant　vertical　loads　and　cyclic　horizontal　loads.　The　failure　phenomenon,　hysteretic　curves,　strains　developing　procedure　on　steel　tubes　and　core　reinforcement　of　the　specimens　with　D/t　of　37.5　and　50　were　analyzed.　The　test　results　show　that,　the　hysteretic　curves　of　CFST　columns　with　large　diameter　are　very　full.　The　energy　dissipation　performance　is　superior.　The　deformation　capacity　is　excellent.　With　the　increase　of　D/t,　the　maximum　deformation　angle　and　the　equivalent　damping　ratio　of　the　specimens　are　slightly　reduced.　The　concrete　crushing　and　tube　wall　buckling　occur　mainly　in　the　region　between　the　bearing　plate　and　the　first　row　of　studs　which　have　a　clear　constraint　effect　on　the　tube　wall　of　the　specimens.　When　D/t　is　relatively　small,　the　steel　pipe　wall　has　stronger　constraint　effect　on　the　core　concrete,　and　the　residual　deformation　is　smaller.　The　core　longitudinal　reinforcement　is　basically　in　the　elastic　state.　The　stirrups　are　far　from　the　yield　strain,　even　they　have　less　constrain　effect　on　concrete.　But　there　is　obvious　slip　between　reinforcement　and　concrete.　The　bending　bearing　capacity　of　concrete　filled　steel　tubular　columns　with　large　diameter　in　the　test　is　significantly　higher　than　calculation　results　according　to　the　relevant　specifications　in　China,　which　means　the　Chinese　specification　is　conservative.　©　2017,　Editorial　Office　of　Journal　of　Building　Structures.　All　right　reserved.