Abstract: To address the problem of source depth estimation when the horizontal distance of the sound source is unknown in shallow water environments, a method based on the depth distribution characteristics of the incoherent acoustic field intensity is proposed. First, the theoretical derivation of the incoherent acoustic field intensity excited by a point source in a horizontally stratified medium verifies the enhancement effect of the incoherent acoustic field at the source depth. Next, the normal mode decomposition is applied to the acoustic field received by a vertical array, and the incoherent acoustic field intensity at different observation depths is obtained based on the amplitudes of each normal mode. When the observation depth includes the source depth, the enhancement effect of the incoherent acoustic field intensity enables depth estimation. Numerical simulations of the incoherent acoustic field are conducted in a shallow water environment with a negative gradient sound speed profile. The results indicate that, regardless of variations in the horizontal distance of the sound source, acoustic frequency, water depth, or sound speed profile, the incoherent acoustic field consistently exhibits enhancement at the source depth. The analysis of sea trial data demonstrates that the proposed method can accurately estimate the source depth, confirming its effectiveness in practical applications. Furthermore, the method exhibits low sensitivity to environmental parameter mismatches but requires a signal-to-noise ratio higher than 0 dB.