Abstract: Discrete trailing edge flap inevitably has gaps when it was manufactured. Regarding influence of gap on aerodynamics performance of wind turbine airfoil, wind turbine airfoil S809 has been used as the research object, and three kinds of the gaps of the S809 airfoil model with trailing edge flaps have been designed. Uniform gap structure has been adopted between the model frontal subject and trailing edge flap. The gap sizes are 1‰c, 2‰c, 4‰c, respectively. In this paper, computational fluid dynamics has been adopted to calculate aerodynamic performance of the airfoil with 10% chord length trailing edge flaps deflected 10° at different angles of attack. The flow field around the airfoil has been theoretically analyzed. The calculated results show that the gap has the effect of reducing the lift coefficient and increasing the drag coefficient of the airfoil. The influence of the gap on flap model is reduced by the increment of the attack angles. When the gap is 1‰c, the pressure coefficient distribution curves and pressure contours of the airfoil model are consistent with the integrated airfoil. The gap between the main body and trailing edge flap has slight impact on airfoil aerodynamic performance. The influence of the gap can be neglected in the aerodynamic performance analysis, and may be used as design parameters.