In order to further improve the resolution of direction-of-arrival (DOA) estimation of antenna array, a high resolution approach of DOA estimation is proposed based on the fourth-order multiple signal classification (FO-MUSIC) approach. First, the antenna array is extended through the fourth-order moment of the received data. Then, the antenna array is extended through the conjugate value of the received data. When the expanded steering vector and the expanded covariance matrix are used for DOA estimation instead of the original steering vector and the original covariance matrix, virtual array elements are formed and the aperture of array is extended. It is shown by simulation results that, compared to the FO-MUSIC approach, the proposed method has a higher probability of target resolution, lower root mean square error and higher resolution. Hence, more virtual array elements are formed by the proposed approach based on twice virtual extension, and the resolution of DOA estimation of antenna array is effectively improved.