Abstract: 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.