With the application of the composition material-concrete-filled-steel-tube (CFST) in civil engineering, the crossing ability of arch bridge gets advanced development. Therefore, it is of interesting to study the stability of such long-span arch bridges, especially which have small width, for the bridge’s safety under long term operation. The present study focuses on studies the transverse stability of long-span arch bridges with small width. To this purpose, a deck type CFST arch-bridge in Gansu Province is adopted as an example. The arch bridge has a span of 180 m, and its width-span ratio is 1/25.7. The finite element method with a special beam element is employed to analyze the stability. The analyses are carried out for the transverse stability of the bridge during construction and under service, respectively. Furthermore, the influence of the spandrel structure and deck elastic restraints to the stability is investigated taking account of geometrical and material nonlinearity.