A design of portable three-dimensional (3D) imaging sonar for both far-field and near-field was proposed, in order to reduce the complexity of 3D imaging sonar system and guarantee the accuracy of beam patterns for the whole underwater scene. The cross array, which could largely simplify the hardware complexity compared to the planar array, was used to achieve 3D sonar imaging and make the system portable. For the transmitting array, a near-field multiple-frequency transmitting algorithm was proposed to guarantee the performance of the beam pattern in the near field. The whole 3D scene was divided into several focal regions according to the depth of field, and the time-delay parameters for each focal region were calculated based on the Fresnel approximation. For the receiving array, an optimized chirp zeta transform beamforming was proposed to implement both far-field and near-field fast beamforming with lower computational requirement than those of other methods. The simulation experiments demonstrated that the angle resolution and the side lobe peak of the proposed algorithm had a relative small variation in the near field focal regions, and the performance of the beam pattern was stable. The real underwater tests verified that the proposed design could generate high quality underwater images within the detection scenes.