To improve the aerodynamic performance of airfoil, a biomimetic fin airfoil is proposed. Taking NACA0018 as an example, a fixed biomimetic fin wing is arranged on the airfoil suction surface, and the relative position and length of the fin wing and their effects on the aerodynamic characteristics of the airfoil and the mechanism of the biomimetic fin wing are analyzed. The numerical results show that the fin wing near the front edge has a significant effect on the large separation flow and the fin wing near the trailing edge has a good effect on the moderate flow separation. The relative length has non-linear relationship with the aerodynamic performance, and when the length is too short, the separation layer cannot be effectively segmented, and if it is too long, the fluid above the separation layer is affected. When the tip of the fin wing just touches the edge of the separation layer, the control effect is the best. The aerodynamic performance of the biomimetic fin wing is determined by the relative position and relative length of the fin wing.