Abstract: InSAR is an effective measurement for generating large scale and accurate digital elevation modeling(DEM).It is a necessary step in the InSAR processing that removing flat earth effect.It makes the phase of interferogram exhibit the contour line of terrain and makes the unwrapping processing easily.DEM can be calculated from interferometric phase without unwrapping processing when the maximum difference of elevation is less than the ambiguous height decided by baseline.A removing flat earth method based on geolocation is introduced in this paper.And its validity is illuminated by test results.