The trailing edge of wind turbine airfoil S822 was modified by adding a Gurney flap and smoothing the lower cambered surface. The aerodynamic characteristics of S822 airfoil and its improved Gurney flap airfoils were studied with k-ω SST turbulence model. The effects of the flap height and starting modification position on the lift and drag coefficients, lift drag ratio, airfoil surface pressure distribution and flow field characteristics were analyzed. The results showed that if the starting modification position was the same, the lift coefficient in certain range of attack angle and the drag coefficient increased with the increasing of the flap height. The lift-drag ratio was the maximum when the flap height was 2% chord length. If the flap height was the same, the lift coefficient and lift-drag ratio increased, and the drag coefficient changed little when the starting modification position was gradually close to the trailing edge. The research can provide references for designing airfoils of wind turbine blade.