Online automatic voltage control（AVC） based on the real time wind speed and power grid status is one of the key technologies for large-scale wind farms to participate in the optimal operation of whole power grid and to improve the voltage profile of local power grid. Based on a three-level wind farm AVC model， an AVC control strategy coordinating the wind farm substation central compensation devices and double-fed wind turbines are discussed. Aiming at optimal AVC of wind farm， a simplified AVC strategy is derived by adopting approximate linearization method based on five-zone diagram， which provides a novel method for practical engineering application. Furthermore， considering the random and fluctuation characteristics of wind energy， the derived strategy is improved to enhance voltage quality and reduce equipment responding times. The simulation results of a wind farm in North China validate the proposed model and stratey.