In this paper, a flatness measurement software was designed called FlatEx including three modules which named absolute testing, inclination measurement of milling straight edges and sub-aperture stitching basing on the wave data by using phase-shift interferometer. By comparison with the process in verification using the flatness interferometer with isoclinic circle fringe, this method proves theoretically feasible for extracting sparse points from dense points in accordance with regulations or unwritten ways. FlatEx can process data from kinds of software in phase-shifting interferometer and bring forward the reasonable requirement result of flatness. By comparison with the results between the phase-shift interferometer and the flatness interferometer with isoclinic circle fringe, it shows a good correlation.