Abstract: The modern interferometry measurment features high precision and non-contact, which makes it widely used. As an essential part of the measruement, apre-processing of interferogram was proposed, and it included edge identification based on mathematical morphology, regional extension based on 2D fast fourier transformation (FFT), as well as unwrapped and wrapped phase based on non-weighted least square method for discrete cosine transform (DCT). At last, surface distribution was obtained. The image aquisition of the SiO2 film and the preprocessing of interferogram was performed.The result indicates that it is consistent with the result measured by Zygo interferometer.