In order to test the radiation sounding performance at the infrared channels of the InfraRed Atmospheric Sounder (IRAS) and obtain the reference calibration coefficients so to offer support to the error analysis of the retrieval products, the ground calibration of satellite onboard instruments is a prerequisite task before launching. Based on the correlation of accepted radiance and output digital counts and the lab vacuum calibration experiment data of IRAS, the relation between radiance and output digital counts is set up by means of the statistical regression method. The lab coefficients at the infrared channels are obtained and the nonlinear responses of IRAS output signals to the input radiance are analyzed. The noise equivalent differential radiance at 290 K temperature is computed and compared with specifications. The results reveal that there is nonlinear response to a certain extent for IRAS, and the nonlinear bias values are less than 0.4 K in the range of 220 to 230 K. The noise equivalent differential radiance meets the functional requirements of the IRAS specifications.