Abstract: Interferometric Synthetic Aperture Radar (InSAR) techniques have been widely used to generate accurate digital elevation model (DEM), and to measure displacement of the earth's surface. In this paper, main principle and development of In- SAR are simply described. How to obtain the 3D terrain information using InSAR technique is summarized. Phase unwrapping for interferometric images is the most important and difficult step in the processing chain, different algorithms published recently are compared, their advantage and disadvantage are analyzed here. The potential development in theory research and application is discussed.