The influence of calibration methods on the radiation measurement precision of a passive Fourier Transform InfraRed (FTIR) spectrometer is studied by an MR-254 FTIR spectrometer from BOMEN Corp.. Taking a high temperature blackbody in the temperature range 50℃ to 1000℃ and a ultra high temperature blackbody in the temperature range 300℃ to 1700℃ as the standard radiators, the spectrometer is calibrated by using linear and nonlinear multipoint calibration methods respectively. The results show that the nonlinear multipoint calibration method is more accurate than the linear multipoint calibration method. The average relative difference between the calibrated radiation spectra and the theoretical radiation spectra is within 0.6%.