In order to reveal the modal behaviors of inflatable fabric beams （ IFB） ,first,the equilibrium shape of IFB is obtained via the form-finding of small Yung＇s Modulus method,with the loading characteristics of flexible membrane structures and the nonlinear geometric factors such as the large deformation and the initial prestress being considered. Thus,the configuration and initial equilibrium stress are sequentially inherited. Next,the modes of IFB are numerically calculated and analyzed by means of the block-Lanczos method,and the effects of pressure, fabric thickness and radius （ slenderness） on the modal behaviors of IFB are analyzed. Then,the wet mode of IFB is proposed and is computed respectivelg via the approximate density method and the fluid-structure interaction method with the consideration of the coupling between the external air fluid field and the flexible IFB. Finally,a comparison between the wet mode and the dry mode of IFB is made. The results validate the coupling effect between the external air fluid and the IFB.