The stalling and flow separation of airfoils at large attack angle directly affect the aerodynamic performance of the airfoils. The measure of setting a tiny sheet around the suction surface of a S809 airfoil at Re=1×10⁶ was discussed by numerical simulation. The length, oscillating amplitude and frequency of the tiny sheet were three main parameters to be considered. A physical analysis of the flow ?elds was performed to illustrate the mechanism of flow control for the improvement of airfoil aerodynamic performance. The results suggest that: when the tiny sheet is stationary, the size of the tiny sheet has a significant influence on the control effect. When the optimal position and size of the tiny sheet have been properly selected, the lift-drag ratio of the S809 airfoil is increased by about 2 times at the angle of attack of 22°. The control effect for flow separation is not good when the smaller size tiny sheet is stationary, but the control effect will be good when it is oscillating: the size of flow separation bubble is significantly reduced, the lift coefficient is increased, and the drag coefficient is effectively reduced, so the aerodynamic performance of the airfoil is further significantly improved.