This　paper　presented　a　test　with　108　squares　pultruded　concrete　filled　GFRP　tube　(CFGT)　columns　which　were　used　to　investigate　the　behavior　of　the　columns　strengthened　with　CFRP　sheet.　Categories　of　failure　modes　were　grouped　according　to　the　failure　characteristics　of　these　columns　under　axial　compressive　loading.　Cracks　of　the　four　edges　in　length　directions　of　GFRP　tube　were　effectively　confined　by　CFRP　sheet.　The　ultimate　bearing　capacity　of　the　columns　with　CFRP　sheet　strengthened　was　greatly　higher　than　that　of　the　columns　without　the　CFRP　sheet　strengthening.　And　the　ultimate　bearing　capacity　was　further　increased　by　increasing　the　GFRP　tube　thickness.　The　ductility　coefficient　and　initial　stiffness　of　the　columns　were　greatly　increased　because　of　the　strengthening　of　CFRP　sheet.　Besides,　the　strain　development　of　ten　square　pultruded　CFGT　columns　under　axial　compressive　loading　was　investigated.　Based　on　the　mechanical　mode　of　square　pultruded　GFRP　tube　confined　by　the　core　concrete　and　the　test　result,　this　paper　proposed　a　calculated　formula　which　was　used　to　predict　the　ultimate　bearing　capacity　of　square　pultruded　CFGT　columns　strengthened　with　CFRP　sheet,　and　a　good　agreement　with　the　test　results　was　achieved.