Measuring the thin film thickness by modern interferometry has advantages of the whole test,high precision and non-contact measurement,the kernel of which is to obtain necessary surface shape and parameter by processing interferogram with reasonable algorithms. The pre-treatment of the interferogram is the most crucial and a basal part,which includes the edge identification of based on mathematical morphology,regional extension based on the 2-D fast Fourier transform (FFT) and unwrapped and wrapped phase based on the non-weighted least squares algorithm for DCT. At the result,surface distribution can be obtained,which lays the groundwork for getting the thin films thickness correctly. In this paper,the image collection of the SiO₂ film and the pretreatment of interferogram is performed. The result indicates that it is basically consistent to the result tested by Zygo interferometer.