To increase the radiometric calibration accuracy of the Hyperspectral Infrared Atmospheric Sounder （HIRAS） onboard FY-3D， the phase-correction module used in the data preprocessing of HIRAS is improved. Phase correction is one of the basic preprocessing steps and is used to determine the zero optical path difference position （ZPD） of the interferogram. The ZPD is the center of the Fourier transform and is also the premise of the Fourier transform； thus， it has an important influence on the inversion spectrum. However， the current phase-correction method used in industry can only accurate ZPD to the integer sampling point. This paper presents the use of the instrument phase method to compare the spectral phases of earth observation， black-body observation， and deep-space observation and extract the linear phase component； thus， the accuracy of the ZPD reaches the subsampling level. A comparison between HIRAS and JPSS-1/CrIS indicates that the improved phase-correction method reduces the mean deviations of the three bands by approximately 0.1， 0.4， and 0.8 K and reduces the standard deviations of the three bands by approximately 0.06， 0.2， and 1.5 K， respectively. Additionally， the dependence of the deviation on the target temperature is reduced. The improved phase-correction method compensates for the shortcomings of the original phase-correction module and effectively reduces the radiation uncertainty of HIRAS.