Based on the study of basic principle of interferometry using 2D FFT arithmetic,a method to measure the length is proposed.The gauge block,as a measured sample,is placed into the light path in a TwymanGreen interferometer,and the interference fringes are generated between the reference beam and the reflected beam of the tested block upper surface and the assistant plane respectively.The interferogram is collected by a image acquisition system.The algorithm processing software is used to achieve block edge identification,region extension,filtering,phase unwrapping etc.The length distribution with block information can be obtained in the whole field and the length of the gauge block can be obtained automatically.The results indicate that the proposed method has advantages of high precision,whole field and noncontact in the measuring.