Inspired by birds raising their feathers to control the separation flow, vortex flap has become one appoach to control the separation flow over airfoils at high angles of attack. In this paper, the aerodynamic characteristics and physical mechanism of vortex flap controlling separation flow over airfoils under different Reynolds number is numerically studied. The results show that vortex flap is able to greatly improve lift of the airfoil at low Reynolds number under high angle of attack. Its physical mechanism is that the vortex flap makes core position of main separation vortex closer to airfoil camparing to original airfoil, and then the low-pressure characteristics caused by th e vortex core will affect the flow field on the upper airfoil surface. In addition, the pressure filed on the upper surface of the airfoil is divided by vortex flap into two parts: the low pressure aera in the front and the high pressure aera at the back. However, under high Reynolds number (corresponding to conventional aircraft Reynolds number), the effect of vortex flap on improving the aerodynamic characteristics of airfoil at high angle of attack is far less effective compared to the case of low Reynolds number. This explains why birds can improve lift characteristics through feather lifting, while vortex flap can only be used as drag plates in conventional aircraft.