Synthetic aperture technology is used in airborne radar or on satellites like SEASAT to obtain high-resolution pictures of the Earth's surface. On the other hand, in the field of underwater acoustics, the effectiveness of synthetic aperture side-looking sonar has long been predicted. However, this has not come to practical use, mainly because of the influence of the trembling motion of the sonar platforms. First, the principle of synthetic aperture sonar and the similarity between CDP stacking of seismic exploration and synthetic aperture processing are discussed. Then, the influence of platform trembling on the synthetic aperture beamforming is quantitatively studied, and the degradation of the picture is evaluated with computer simulation to show the allowable range for the trembling motion amplitude and period against the used signal frequency.