Amethod for measuring internal stray radiation in thermal infrared spectrometer system was proposed in this paper,which is based on radiometric calibration of a detector and a thermal infrared spectrometer. The output response curves of detector and the spectrometer system to blackbody radia-tion energy of asingle spectral channelare calibrated separately. Therefore,the internalstray radiation grayvalueandradiation fluxvalueofthespectrometer can be obtained andthegrayvalueand radiation flux of the internal stray radiation at different integration times and temperatures in the system can also be calculated. The proposed method was used to measure the internal stray radiation of an existing spectrometer,and comparative experiments were carried out. Results show that the error between the experimental results and the theoretical predicted values is less than 1%. The proposed method has high maneuverability and can be used to measure the proportion of internal stray radiation in total out-put DN value of the system,predict the influence of refrigeration on internal stray radiation of spec-trometer, and measure the inhibition effect of other internal stray radiation suppression methods.