Abstract: The basic principle of depth estimation of an underwater sound source with vertical array in the direct sound region in deep sea is analyzed firstly. Through frequency-domain broadband beamforming, which involves coherent superposition of the direct sound and surface reflected sound, the depth information is obtained along with the output envelope of the beam. It could be allowed for depth estimation of the sound source. Finally, interference oscillation period matching is employed to reduce errors caused by an inhomogeneous sound speed profile. On this basis, simulation is conducted to examine how the sea surface environment affects source-depth estimation performance. Since sea surface roughness impacts the propagation of sea surface reflected sound, its influence on depth estimation is investigated. By adjusting wavelength and amplitude of ocean wave through simulation experiments, it is found that this method exhibits good stability when sea state is below class 5. Lastly, through analysis and comparison of received spectrum levels of merchant ships at different distances with received spectrum levels of sound source at fixed array observation angles, it was discovered that improving beamforming methods can effectively suppress long-distance interference. The research results provide valuable insights for parameter design considerations regarding deep-sea vertical arrays and contribute to further research on target depth determination.