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Abstract. The difficulties of phase unwrapping was increased with the existence of flat-earth phase removal, at the same time ,it covered up the real topographical conditions .In order to solve this problem ,the paper presented a method of re-flattening. based on DEM to eliminate flat-earth phase, with the interferogram, and then chose some special control points to construct polynomial fitting, removed residual phase and noise. By analyzing the ALOS/PALSAR at the mine subsidence area, the contrast experiment was carried out. The results showed that the method can clearly reflect the earth′s surface changes, reduce the surface deformation error, get a more accurate interferogram phase and improve the precision of the INSAR interferometry. SyntheticAperture Radar Interferometry (INSAR)measurement technique combined the synthetic aperture radar imaging theory and electromagnetic interference technology, using the observations of two or more SAR image complex data in the same area to process coherence to obtain more precise digital elevation model and more accurate millimeter surface deformation information [1]. However, Flat-earth phase removal had a very important effect on the entire INSAR data. Due to the using of synthetic aperture radar imaging with slant range way. Flat-earth phase removal appeared, so the distance was different when the radar echo signal propagated, and phase position changed which should remain unchanged at the same height in the interference figure, the phenomenon also showed that the interferogram presented periodically interference fringes.The presence of flat-earth phase removal obscured the change of interference fringes, so the interferogram can not reflect the real situation of geomorphology, and made fringes intensive, so the difficulty of phase unwrapping increased. There fore, before unwrapping the interferogram, the flat-earth phase must be removed. Approaching to common flat-earth phase removal were mainly the following methods[2-3]: (1) A method of flat-earth phase removal based on orbit parameters was to eliminate flat-earth phase that the reference ellipsoid caused by the position and orbital parameters of the imaging area. The principle of the method was simple and high precision theory but lowering efficiency, which depended on the accuracy of the orbit position.(2) Frequency shift method had a small amount of calculation and no other additional conditions merit. Tocalculate the flat phase by frequency interference fringes, eliminate the flat-earth phase. It was more effective for flat areas, But for the settlement of the surface area, the fringe frequency that some part of terrain changes was as same as peaceful fringes of the frequency, if it was removed. It was likely to cause an error, and can notaccurately eliminate the influence of the flat phase. (3) Flat-earth phase removal based on DEM method can achieve high elevation accuracy under the conditions of little parameter data. However, this method was influenced by abnormal phase and phase noise, made some difficult ...