Abstract: Taking the airfoil NREL S809 as an object of study, numerical simulations were conducted to investigate the effects of flap angles on the aerodynamic performance of the airfoil at different Reynolds numbers, while the aerodynamic parameters of unsteady flow field were analyzed with power spectrum. Results show that the optimal flap angles are approximately linear with respect to the angles of attack at different Reynolds numbers. The fluid has stronger attaching ability to the airfoil surface at higher Reynolds numbers, making the optimal flap angles get smaller at corresponding angles of attack. The flap at high Reynolds numbers has stronger ability to reduce the fluctuation amplitude of aerodynamic parameters than that at low Reynolds numbers.