Abstract: The stability of a pinned plate-type beam in a rigid rectangular tube with incompressible viscous fluids flowing within it is studied. The assumption mode method is utilized to derive the motion equations of the structure. The effects of the thickness of the beam and the gaps between the beam and the tube walls on the stability of the model are discussed. The results show that both the thickness and the gaps have significant influences on the stability of the system and the critical velocity of the system changes remarkably with their variations. In addition, whether the two gaps change at the same time, that is, one changes while the other keeps unchanged, has different effects on the critical velocity of the system.