The geostationary interferometric infrared sounder (GIIRS) is a key payload on the geostationary FengYun-4B meteorological satellite of China. Because the GIIRS can measure atmospheric upwelling hyperspectral infrared radiance, it can be used to retrieve atmospheric temperature and humidity profiles and can also be applied to numerical weather prediction models. For the prediction of GIIRS’ post-launch performance, a series of blackbody calibration experiments are performed before launch in a laboratory thermal vacuum tank in a ground laboratory to test the radiometric performances of the instrument, including sounder sensitivity, radiometric calibration accuracy, and dynamic observation range. The noise equivalent differential radiance (NEdR) in the long-wave infrared (LWIR) band is less than 0.5 mW/(m ²·sr·cm ^-1), and that in the mid-wave infrared (MWIR) band is less than 0.1 mW/(m ²·sr·cm ^-1). Both NEdRs meet the sensitivity design requirements. As for the radiometric calibration, the average calibration difference in the LWIR band is improved from 1 K to 0.2 K after the nonlinearity correction. In addition, in the observation range of 220--315 K, the calibration differences meet the design requirement of 0.7 K. The MWIR band is susceptible to noise when the instrument is used to observe low-temperature targets. Nevertheless, the calibration differences also meet the 0.7 K index requirement in the dynamic range of 260--315 K.