Recent researches show that an automatic moving deflector(AMD) attached to the airfoil and 3D vehicle model can enhance the delay of boundary layer separation and a decrease in drag. In this research, a parametric study is performed with length and tilting angle, which are the main parameters of AMD, through wind tunnel experiments to analyze the aerodynamic characteristics of AMD attached to the NACA4412 airfoil. Results show that the airfoil model with smaller length or tilting angle of AMD has a higher lift-to-drag ratio and a smaller magnitude of pitching moment coefficient. The delay of stall is also verified. However, if the length of AMD is less than a critical value, AMD has no significant effects on the delay of stall.