Abstract: Aiming at active target depth estimation in shallow sea, a single sound source is used to transmit low-frequency broadband signals, vertical dual receiving hydrophones receive target echoes, and the phase characteristics of the broadband target echo ratio are used for matched phase processing. The method not only eliminates the influence of the target scattering characteristics on the depth estimation of the active target, but also only needs to calculate the one-way sound field, and the calculation amount is small, which is conducive to the realization of real-time processing requirements. At First, through theoretical derivation, a sound field feature with implicit target depth and independent of target scattering characteristics---one-way propagation vector broadband interference structure is defined. The relationship between the order of the normal wave and its amplitude and phase is simulated and analyzed under the condition of Pekeris waveguide, and it is found that selecting more one-way propagation vector broadband interference structures contributed by the order of the normal mode can improve the performance of active target depth estimation. Furthermore, the simulations show that under the condition that the transmitted signal selects a certain time length and bandwidth, the mean error of active target depth estimation is less than 5 meters when all orders of normal mode are used and the signal-to-noise ratio is greater than -10 dB. Finally, the influence of signal time length and bandwidth on active target depth estimation performance, and the method's robustness in case of bottom sound velocity, sea depth and sound velocity profiles mismatch are analyzed.