Theoretical modeling is effective to study the flow aroused by wings of dragonfly in flapping flight, which is highly unsteady, i.e., characterized as St>>1/Re. The aerodynamic effects of a model wing with time-changing deformation during cyclic wing beats in dragonfly forward flight are investigated, using two-dimensional panel method. In this study, the dragonfly wing is modeled as a two-dimensional deformable plate, and the panel method is improved to deal with vortex shedding from both leading- and trailing- edges of the plate. Results show that lift is enhanced greatly during down-stroke stage of the flexible wing, compared with that in the rigid case, and the actual amplitude of the wing deformation is contributive to reduce the mean aerodynamic power and enhance lift, which means a longer-time forward flight is available.