Abstract: Due to the coupling between large prescribed motions and flexible deformation, classical dynamics theory could not be applied in the flapping wing aeroelastic studies. In this paper, a dynamic fluidstructure coupling computational framework is developed which is able to simulate the complex flow around flexible flapping wings and investigate the effects of the wing flexibility on aerodynamic performance. The equations of motion are derived using Hamilton's principle, and a Newmark solution method is used to solve the above equations which takes into account of inertial force and aerodynamic force together. The aerodynamic force is obtained by RANS (Reynolds-Averaged Navier-Stokes) solver. An IPS (Infinite Plate Spline)method is used for data exchange between aerodynamic and structural grids. A loose CFD/CSD coupling method is used, and it only requires 3-4 sub-iterations to get converged. A NACA0012 crosssectional rectangular wing with prescribed pure plunge motion is investigated. Both rigid and flexible wing results are presented, and good agreements between experiment and computation are shown regarding tip displacement and thrust coefficient.