Abstract: InSAR (Synthetic Aperture Radar Interferometry) is a kind of earth observation technique. Due to its working performances of high precision, wide coverage, penetrability, all-weather and all-time, InSAR owns unique advantages on digital elevation model (DEM) extraction and surface deformation monitoring. During the image processing of InSAR, phase-unwrapping plays an important role, whose results directly determine the final precision of DEM. This paper, using ASAR data, with the ROI_PAC software, briefly analyzes the DEM abstraction. The ROI_PAC adopts the classical path following algorithm of branch-cut method for phase-unwrapping. Based on a detailed analysis and discussion on this classical algorithm, the authors propose a new algorithm improving method for residual point searching.