Abstract: The equations for fluid forces acting on a flat beam in a rigid water trough with axial flow were derived using the analytical equations for fluid dynamic pressure of typical plate-fluid-plate model. The natural frequencies of a hinged 4-span flat beam in air and in axial viscous fluid flow were calculated with the assumed mode method. The results show that the relative error between the calculated 1st order natural frequency and the experimental one is 1.7%,which satisfies the need of engineering application. The natural frequencies decrease with the increase of the fluid velocity,the length and width of the beam,and go down to zero when fluid velocity increases to a large value. The dynamic viscosity of fluid has little effect on natural frequencies.