The Geostationary Interferometric Infrared Sounder (GIIRS) on board FY-4A is a Michelson interferometer infrared sounder. The interferogram is sampled at discrete points. The zero path difference (ZPD), which is the peak of the interferogram, is not sampled. This effectively shifts the interferogram and adds a linear phase to the spectrum. A linear regression on the residual phase of the calibrated spectrum will reveal the shift. This method has been used in the interferogram signal processing of the GIIRS, and has obtained a better results.