Different from the traditional rigid body WECs, a flexible WEC consisting of flexible material is numerically modeled in this paper. A fully coupled fluid structure interaction solver that can cope with the dynamic interplay between the flexible membrane and the surrounding fluid is developed. The fluid field is resolved by computational fluid dynamics (CFD) method, where a multi-phase flow solver is chosen to simulate oceanic waves. The flexible body is modeled as a two-dimensional beam. The data exchange between the two solvers is achieved by a coupling strategy. With this tool, the various flexible deformations can be captured with the wave propagating. The detailed flow field is obtained in the vicinity of the membrane along with the dynamic force acting on it. Although a linear structure model is used, the CFD results qualitatively repeat the experiment data. Our results show that the incident wave period influences the membrane deformation. In addition, adding a collector at the top of the membrane can increase the extent of deformation attributed to the increased wave excitation force.