Abstract: According to the requirements for the absolute calibration of spectrometer, a diffusing source formed by integration sphere was analyzed. The theoretical model of the spectral irradiance at the exit of an integration sphere was established and analyzed with a spectrometer. The calculation and experiment results confirmed the validity of the model. In the band of 0.6m～0.85m, the calculated value obtained from the model has an error of less than 4% compared to the measured value; at the wavelength of 0.9m, the error is 8.65% due to response property of the spectrometer itself and the blackbody for calibrating the spectrometer. It is showed that the model can be used to calculate the radiation value of integration sphere source.