Abstract: The plastic limit analysis method is proposed to study the ultimate load-carrying capacity and failure modes of long-span cable-stayed bridges with floating systems under the action of vertical loads. Based on the assumptions of the plastic limit analysis and on the elementary mechanisms, the feasibility of the method applied to the failure analysis of cable-stayed bridges was proved. Combined with a single-tower cable-stayed bridge with a floating system, the calculation formula of ultimate load-carrying capacity was derived and the corresponding failure modes are analyzed. The plastic collapse loads and the corresponding failure modes of the cable-stayed bridges were obtained through the interior point method which is an optimization method, and then the comparison analysis of two results respectively from the plastic limit analysis method and the OpenSEES were conducted. The results show that: the girder of cable-stayed bridges subjected to vertical loads presents the characteristic of global failure when the cable's yield strength is relatively small, otherwise it presents the characteristics of local failure. The influence of the girder's yield strength on the failure behavior is just the opposite of those cables. The cable corrosion not only reduces the ultimate load factor of the cable-stayed bridges, but also changes its failure modes. Compared with the FEM, the plastic limit analysis method is simple and easy to operate. It can be used to study the ultimate load-carrying capacity and failure modes of cable-stayed bridges subjected to vertical loads and can quickly and effectively evaluate the influence of changes of structural parameters on the failure behavior of bridges.