Abstract: A delay optimization design method for near-field focused beamforming is studied to broaden the visual field of the three dimensional imaging sonar with a planar array. In this method, the first three terms in the Taylor series of the exact near-field delay expression are selected and weighted under the new definitions of the azimuth and pitching angles. By setting the partial derivatives of the three weighting coefficients equal to zero, the optimum weighting coefficients can be obtained. Therefore, the difference between the delay expression of the three weighted terms with optimum weighting coefficients and the exact near-field delay expression is minimal. In other words, the delay expressed by the three weighted terms with optimum weighting coefficients is very close to the exact near-field delay, which can be used effectively to broaden the visual field of imaging sonar. Finally, simulation for a planar array is conducted to verify the effectiveness of the proposed method in this paper.