Abstract: For the weak line spectrum target detection in the complex underwater environment, this paper proposes an enhancement detection method in the frequency-wavenumber domain. This method utilizes the spatial and temporal coherence characteristics of line spectral targets to construct a two-dimensional joint filter adapted to passive processing in a uniform linear array, and achieves two-dimensional enhancement of line spectral targets through adaptive iterative methods. On this basis, the main lobe detection accumulate result in the frequency-wavenumber domain is used as the output of the direction spectrum to achieve high direction resolution for line spectral targets. Simulation results show that the processing gain of this method for line spectral targets with a signal-to-noise ratio of −35 dB within the bandwidth can reach 21.3 dB, with an direction resolution within 0.3°. Using real data to test this method, the processing gain for line spectral targets can reach 8 dB. Compared to broadband processing methods such as the CBF and SSED, this method exhibits a higher gain in processing line spectrum targets; compared to line spectrum processing methods such as the ALE and STJD, this method has lower sidelobe fluctuations and higher target azimuth resolution.