Abstract: The quality of beamforming is the key to the imaging quality of underwater multi-beam imaging system. When the signal frequency is constant, the conventional phase-shifting beamforming algorithm can only get a narrower beam by increasing the aperture of the array, but this method is restricted by practical conditions in engineering. Therefore, a virtual array beamforming algorithm based on least absolute deviation (LAD) estimation is presented, by which a narrower and more robust beam can be obtained without increasing the array size, moreover the array gain and the angle resolution of sonar image can be increased under low signal to noise ratio (SNR). The superiority of the above method is verified by using Matlab simulation and comparing with other methods.