Abstract: In the studing of aerodynamic performance on DU series airfoils,the drag and lift of DU97-W-300 airfoil were found unsteady.By comparing variable graphs against time of drag and lift for many kinds of incidence and thickness of trailing edge,it was found that boundary-layer separation had something to do with this phenomenon,and thickness of trailing edge had vital influence on boundary-layer separation.The study result show that there is a critical value of airfoil's trailing edge thickness,when the trailing edge is thinker than the critical value,the lift and drag will be unsteady,otherwise when thinner than the critical value,they will be steady.