Abstract: To address the snapshot deficiency in large-aperture arrays for low-frequency long-range passive underwater detection, this study proposes a hierarchical processing architecture that alleviates effectively the performance degradation due to snapshot-deficiency of spatial correlation matrix estimation. The proposed method first forms directional conventional beams using compact subarrays and then adaptively combines the subarray outputs. Theoretical analysis reveals that spherical-wave signal mismatch mechanisms are leveraged to suppress near-field strong interference, with white noise gain (WNG) constraints ensuring dynamic balance between interference mitigation and desired signal preservation. Experimental validation using sea-trial data demonstrates that a 128-element array can completely suppress a 30 dB near-field interference with WNG set below −3 dB and 1−3 snapshots per subarray, and reveal obscured and sidelobe-masked weak targets.