To improve the ability of detecting underwater tar-gets under strong wideband interference environment, an effi-cient method of line spectrum extraction is proposed, which fully utilizes the feature of the target spectrum that the high intense and stable line spectrum is superimposed on the wide continu-ous spectrum. This method modifies the traditional beam form-ing algorithm by calculating and fusing the beam forming results at multi-frequency band and multi-azimuth interval, showing an excellent way to extract the line spectrum when the interference and the target are not in the same azimuth interval simulta-neously. Statistical efficiency of the estimated azimuth variance and corresponding power of the line spectrum band depends on the line spectra ratio (LSR) of the line spectrum. The change laws of the output signal to noise ratio (SNR) with the LSR, the input SNR, the integration time and the filtering bandwidth of dif-ferent algorithms bring the selection principle of the critical LSR. As the basis, the detection gain of wideband energy integration and the narrowband line spectrum algorithm are theoretically analyzed. The simulation detection gain demonstrates a good match with the theoretical model. The application conditions of all methods are verified by the receiver operating characteristic (ROC) curve and experimental data from Qiandao Lake. In fact, combining the two methods for target detection reduces the missed detection rate. The proposed post-processing method in 2-dimension with the Kalman filter in the time dimension and the background equalization algorithm in the azimuth dimension makes use of the strong correlation between adjacent frames, could further remove background fluctuation and improve the display effect.