The　concrete-filled　steel　tubular　(CFST)　composite　column　in　this　paper　is　a　new　type　of　structure　which　is　composed　of　four　CFST　members　connected　together　by　reinforced　concrete　(RC)　plates　in　the　main　loading　direction.　The　axially　loaded　specimens　fail　due　to　the　crushing　of　RC　plate　near　the　mid-height　section　while　the　CFST　longitudinal　members　don＇t　reach　their　strengths.　Therefore,　the　ultimate　strength　of　CFST　longitudinal　members　should　be　reduced　when　the　superposition　method　is　applied　to　calculate　the　axial　ultimate　strength　of　CFST　composite　strut　column.　The　portion　of　total　load　carried　by　the　CFST　longitudinal　members　corresponding　to　the　crushing　of　RC　plate　is　derived　based　on　the　deformation　compatibility　equation,　and　the　ratio　of　the　portion　of　the　total　load　resisted　by　longitudinal　members　to　the　ultimate　strength　of　CFST　member　itself　is　defined　as　the　reduction　coefficient　of　the　load　carrying　capacity　of　CFST　longitudinal　members　in　CFST　composite　strut　column　subjected　to　axial　loads.　The　specified　value　of　reduction　coefficient　based　on　different　calculation　theory　was　discussed　using　finite　element　method,　and　the　effect　of　confining　factor　on　the　value　of　reduction　coefficient　was　also　analyzed.　The　results　show　that　the　reduction　coefficient　value　is　constant　for　various　specimens　with　different　confining　factor　when　the　unified　theory　is　used,　while　the　reduction　coefficient　from　the　limit　equilibrium　theory　decreases　linearly　with　the　increase　of　confining　factor,　and　the　relationship　can　be　fitted　simply　as　a　linear　function.