The kernel of modern interferometry is to obtain necessary surface shape and parameter by processing interferogram with reasonable algorithm. The circular interferogram must be spread into square region when interferogram is disposed by using 2-D FFT arithmetic, which is applied only to numeric disperse data and requires square distributing region.On the basis of the study of the basic principle of in- terferometry by using 2-D FFT arithmetic, the principle and method of interferogram region spreading is proposed.This technique has been used in region spreading of an actual interferogram with satisfactory re- sult. The results indicate that the method has high processing precision and establishes the foundation to phase recovery.