In the interferometric SAR (InSAR) processing, the interferogram flattening is a key procedure for eliminating the flat-earth phase and decreasing the density of fringes. In this paper, two different techniques for flattening, using respectively the orbit data and the interferogram spectrum, are analyzed. Various error sources, their comparisons and especially their influence on final DEM accuracy are discussed in detail. According to the sample data experiment, several conclusions can be drawn: the flattening algorithm based on geometry parameters with precise orbit data can remove the flat earth phase accurately and constrain the DEM error to a low level, which is obviously better than that based on interferogram spectrum. When the mean spatial frequency of the interferogram spectrum is equal to zero, relatively low DEM error will be retrieved with the flattening algorithm based on interferogram spectrum. However, if precise orbit data cannot be obtained, DEM reconstruction can’t meet high accuracy requirement with the flattening algorithm based on geometry parameters or interferogram spectrum.