Abstract: Even though slotted airfoils can effectively inhibit flow separation on the airfoil suction surface and thus improve the aerodynamic performance during stall, they may undermine the aerodynamic performance at small angles of attack. In this paper, we studied the influence of slot parameters on the aerodynamic characteristics of airfoil S809 utilizing CFD (Computational Fluid Dynamics) simulations, with the aim to design an adjustable slot that can realize significant improvement of aerodynamic performance at both small and large angles of attack. Exploiting numerical simulation results yielded a base slot configuration that can considerably improve airfoil’s stall characteristics, with the maximum lift coefficient increased by 70.8% and the stall angle of attack delayed to 21°. By translating and rotating the basic slot configuration, we obtained a trapezoidal curved slot that allows slight adjustment and two slot configurations that can maintain the airfoil’s aerodynamic characteristics at small angles of attack. Moreover, based on the geometric relationship between the basic slot configuration and the one rotated anticlockwise by 5°, we proposed an adjustable slot configuration that can significantly improve the stall characteristics and does not undermine the aerodynamic characteristics at small angles of attack. The proposed adjustable slot configurations shed some lights on designing slotted airfoils.