Due to the nature of lightweight and flexibility of micro flapping wing structures, elastic deformation as a result of aeroelastic coupling is inevitable in flapping motion. This effect can be significant and beneficial to the aerodynamic performance for a flexible flapping wing versus a rigid one. To characterize the effect, a two dimensional quasi steady aerodynamic model (QSAM) based on blade element theory has been extended from previous study. Both twist motion and twist flapping coupled motion are included in the traditional flapping model to fully characterize the aeroelastic effects of flapping wing coupled motion. In the flapping and twist stages, spanwise and chordwise deformation angles are introduced to characterize the flexibility effect of flapping wings. A regular function of deformation angle variation with time is created to modify the shape variables of flapping wing partition units at different times. Secondly the velocity of flapping wing partition units is analyzed to characterize the inflow and combined velocity of flapping wing motion when flexible deformation occurs. Finally the model is applied to aerodynamic analysis of a flexible flapping wing with aeroelastic coupling effect. Significant increase of lift and thrust coefficients can be achieved for a flexible flapping wing by the spanwise deformation, but twist deformation can have negative effects on lift and thrust during flapping and twist coupled motion of flapping wings.