A method of making design attack angles as a variable was presented. Considering the wind speed probability distribution of wind farm, an optimization model with the objective of maximum annual energy production was established in the range of rated power and basic operating wind speed. The blade geometry of a 20kW constant-pitch wind turbine was optimized by using genetic algorithm according to the model. Through the evaluation of the aerodynamic performance of 20kW constant-pitch wind turbine and the comparison with the same type wind turbine of NREL, the results showed that optimized wind turbine had a better aerodynamic performance, which illustrated the advantages of the method in the design process of constant-pitch wind turbine.