Abstract: It is clear that the flexible deformation of a flapping wing has strong influences on its aerodynamic performance. However, most of previous numerical studies focus on rigid flapping wings. In this paper, a numerical study on the effect of flexible deformation of a flapping wing on its aerodynamic performance is conducted. A simplified two-dimensional flexible flapping wing model is used. The fluid-structure interaction flow is simulated by solving the N-S equation using the finite volume method coupled with a solver to determine the deformation of the flapping wing with a finite element method. The results show that the flexible deformation of the flapping wing changes the vortex structure around the flapping wing significantly. Appropriate deformation can delay the shedding of the leading edge vortex, and hence improve the propulsive efficiency of the flapping wing, but the wake capture mechanism that leads to high lift force under low Reynolds number is weakened.