Abstract: An array aperture extension method based on covariance matrix fitting is proposed to improve the resolution of uniform linear array with small aperture. The relationship between the covariance matrices of signals received by arrays with different number of array elements is analyzed. According to this relationship, an array extension optimization algorithm is constructed to fit the covariance matrix of a large aperture virtual array by using the covariance matrix of a small aperture actual array. Simulation and lake experiment results show that using the fitted covariance matrix in existing DOA estimation methods can reduce the beamwidth and improve the resolution. As the number of virtual array elements increases, the resolution synchronously improves. This method can be used to improve the DOA estimation performance when the array aperture is insufficient or the signal-to-noise ratio is reduced.