Abstract: Flapping-wing micro air vehicle (FMAV) is one of the most important research directions of future aviation. The bird or insect in nature possess the advanced ability of flying because of thousands of years' evolution, so the study of their flight kinematics as well as flow mechanism can greatly contribute to the development of FMAV. To investigate the unsteady aerodynamic mechanism of flapping wing, simplified plunging/pitching motion of a rigid 2-D S1223 airfoil is firstly taken into consideration. Numerical method is based on the hybrid dynamic mesh technique and unsteady flow solver. To improve the moving ability of dynamic mesh, the radius basis function is used to calculate the displacement of volume nodes. For unsteady flow solver on moving mesh, geometric conservation law is satisfied by constraining the normal velocity of cell surface. Second-order finite volume method is used. Dual-time stepping method and BLU-SGS implicit scheme are adopted for time marching. The unsteady lifts, thrusts as well as power consumptions with different flapping parameters are abtained. The flow mechanism for lift and thrust is studied. The influence of down-stroke time ratio and stroke angle is analyzed, which would greatly influence the aerodynamic performance and the power consumption. Numerical results show that the 'static effect' of the airfoil plays the main role for lift generation.Meanwhile, the unsteady flow induced by the plunging/pitching motion can enhance the lift by weakening the flow separation and increasing the equivalent angle of attack. The maximum lift per power consumption can be got when down-stroke time takes about 65%-70% of the whole stroke cycle, which agrees well with observation data. Further, a set of appropriate flapping angles are specified based on these analyses, which would greatly contribute to the realistic 3-D simulations in the future.