The effects of winglet's sweepback angle on civilian airplane's aerodynamic performance at three different flight conditions, taking-off, cruising and landing, are studied with using CFD method. The result shows that for the simplified airfoil in this paper, the regular pattern of lift-to-drag is first increased then decreased with the increase of winglet sweepback angle in all the three different flight conditions. And it also abtained that the best sweepback angles in the three flight conditions, taking-off, cruising and landing, are 30°, 50°, 40°, in a 10°-step calculation condition. The conclusion has certain reference value to the study of the variable winglet in the future.