Abstract: In order to improve the passive positioning accuracy of three-element array, a modified cross- spectral algorithm is proposed for time delay difference estimation. The improved method solves phase ambiguity issue in conventional the cross-spectral method, and enhances the robustness of cross-spectral method in practical application. The time delay difference obtained with the proposed method contains cycle value and correction value. The cycle value is obtained by compensating the phase of cross-spectrum signal, and the correction value is obtained by applying least-squares to cross-spectrum signal after compensation. Theoretical analysis and experiment results show that this method can achieves the evaluation accuracy of10-6 s for sampling rate of 103 Hz, and has a strong practicability. Therefore it can effectively improve the positioning accuracy of three-element array passive positioning.