Experiment　study　on　a　CFST　(single　tube)　arch　with　one　rib　under　spatial　loads　is　carried　out.　The　study　shows　that　the　behavior　of　the　arch　under　in-plane　loads　is　more　sensitive　to　imperfection　than　that　under　out-of-plane　loads.　The　lateral　deformation　is　one　of　the　main　control　factors　for　the　ultimate　load-carrying　capacity　of　the　arch.　Compared　to　CFST　arch　under　in-plane　loads,　the　geometrical　nonlinearity　becomes　the　dominant　factor　in　the　nonlinearity　and　ultimate　load-carrying　capacity　of　the　arch　while　the　material　nonlinearity　turn　to　be　a　subordinate　factor.　The　dual　nonlinearity　FEM　of　CFST　arch　under　spatial　loads　analyzed　with　general　finite　element　software　is　discussed　as　well.