We present a numerical simulation of the interaction between laminar flow and a highly flexible elastic sheet.The mathematical model for the simulations includes a finite-volume based fluid solver for incompressible viscous flow and the combined finite-discrete element method for the deformation of solid.An immersed boundary method is used to provide a deforming boundary for the fluid and applies forces to the boundary nodes of the solid.The simulated results for both swing pattern and oscillation frequency of the elastic sheet in low Reynolds number flow compare well with experimental data.