Abstract: A broadband downward-looking multi-beam 3 D imaging algorithm in frequency domain is proposed.Echo in array-element domain is transferred into wave number-frequency domain through 2 D Fourier Transformation.Vertical match filtering and phase compensation of broadband signal are subsequently applied.The result is then transferred into array-element domain through 2 D Inverse Fourier Transformation to complete the across-track-vertical imaging processing.Synthetic aperture method is used for along-track processing to obtain the 3 D image.Computer simulation and sea trial results demonstrate that across-track resolution has obvious improvement and the side lobe level is significantly reduced of 10~30 dB compared with the conventional narrowband downward-looking multi-beam 3 D imaging algorithm.Compared to the traditional broadband beamforming,the computational complexity of the presented method is significantly reduced,which is more suitable for real time application.