The concept SAS (synthetic aperture sonar) comes from SAR (synthetic aperture radar) which is one of the most advanced imaging radar so far. With the development of sonar platform motion-control techniques and motion compensation algorithms, the research on SAS is attracting more and more attentions. However, SAS imaging is mostly used in topographic survey at present because the imaging algorithms are almost all for 2-D circumstance. But the 3-D target and its information such as the height, the posture and so on are much more important in many applications. A 3-D target height extraction method (3-DTHEM) based on SAS 2-D imaging is proposed in this paper. The simulation results show that the proposed method could image a 3-D target and then extract the height information of the target.