The Geostationary Interferometric Infrared Sounder （GIIRS） on board FY-4A is an infrared Fourier transform spectrometer. The primary use of GIIRS is to support numerical weather predictions by providing improved temperature and water vapor profile information, which requires high radiometric precision and accuracy. To meet the requirement, post-launch GIIRS calibration and validation activities were carried out. The spectral calibration has been done through determining the effective sampling frequency which is to take advantage of gaseous line center positions known to high accuracy. The radiometric calibration has been performed through using observation of hot and cold black body reference sources as the basis for two-point calibrations at each wavenumber. The amplitude of some spectrum bands was reduced, which may be caused by some materials gradually releasing a little gas. The estimated absolute spectral calibration uncertainty is 10 ppm, and the mean brightness temperature differences between GIIRS observations and IASI observations are all less than 1 K in the long-wave and mid-wave band.