A method 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 radiation energy of a single spectral channel are calibrated separately. Therefore, the internal stray radiation gray value and radiation flux value of the spectrometer can be obtained and the gray value and 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 output DN value of the system, predict the influence of refrigeration on internal stray radiation of spectrometer, and measure the inhibition effect of other internal stray radiation suppression methods.