In order to understand the effect of relative thickness on aerodynamic characteristics of wind airfoils, three airfoils with different thickness of NACA4412, NACA4415 and NACA4418 were studied. The influences of relative thickness of airfoils on lift and drag coefficient, lift-drag ratio, flow field and pressure coefficient of airfoils were analyzed. The results show that the lift-drag ratio of airfoils decreases with the increase of thickness when the angle of attack is small. At large angles of attack, NACA4415 and NACA4418 airfoils still have high lift-drag ratios, while NACA4412 airfoil has lower lift-drag ratio. With the increase of attack angle, three airfoils all exhibit airflow separation and the formation of tail vortex. NACA4412 airfoil has a larger tail vortex distribution than NACA4415 and NACA4418 airfoils. The results provide a reference for the design and optimization of wind turbine blades.