Abstract: The stability of a four-span simply-supported plate-type beam in an axial viscous flow was studied. The coupling effect of fluid-structure was considered based on viscous fluid theory. The differential equation of system was presented based on the assumption method and changing tendencies of the critical velocity for the flutter with both the gap and the thickness of the beam were discussed. The result shows that the system loses stability by the flutter. The critical velocity for the flutter decreases with the increase of the gap while increases with the increase of beam’s thickness.