A design method of wind energy conversion system based on the concept of power factor is proposed. The optimal power energy and electricity quality are considered as optimization objects. The blades are extended to improve power out-put at low wind speed and shrunk with regulating mechanism at high wind speed to adjust power out-put. Taking the rated wind speed, the maximal length of an extended blade, the minimal length of a shrunk blade and the central radius of a blade as optimization variables, an optimization model of the wind turbine is proposed to obtain optimal design parameters using genetic algorithm. Finally, the availability of optimal design of wind energy conversion system is validated by a case of wind turbine. 