Duckbill valves (DBVs), made of a fabric reinforced layered rubber composite, are extensively used for nonreturn axial water flows with low back pressures. Fluid–structure interaction (FSI) is directly involved in the opening process of the DBV, with the opening depending on the pressure differential across the valve. In this paper, a simplified FSI model of the DBV is presented using a finite element method (FEM). The valve is modeled as a laminated thick shell structure with some simplifications to the boundary conditions. The pressure load acting on the shell surface of the DBV is a function of the variable valve cross-sectional area and determined, for preliminary analysis purposes, by using a simple potential flow model for the fluid mechanics. The hyperelasticity of the rubber and orthotropy of the fiber reinforcement, as well as large deflections of the DBV, are considered in the simulation. The valve is modeled as being closed when the upstream pressure is applied, and the transient opening process is tracked until a steady state opening is achieved. Several static cases of viscous flow passing through the deformed valve structure have also been carried out to compare the pressure and velocity fields of fluid flow with the corresponding pressure and velocity distribution predicted by the simplified model and to compare the hydraulic performance of the DBV predicted by both models.