Abstract: In pracital underwater passive signal processing， weak target be interested in only in certain frequency component spectrum processing shows relatively obvious spectral characteristics， the existing post-processing method based on energy accumulation exist spectrum energy is less than the noise of the target is a smooth， eventually led to be the target cannot be effective detection. In order to solve this problem， a new adaptive beamforming post-processing method is proposed in this paper， in which the spectrum of interest is extracted from the beam signal spectrum by Eckart optimal filter， and the weighted value of the beam spectrum is obtained. Thus， the weak target signal can be enhanced. The proposed method is verified by the actual towed array and the flank array data. According to the experimental results， the proposed method can obtain a weak target detection gain of about 3 dB for weak targets in the shallow sea multi-interference environment.