Stray radiation is an important indicator for the infrared optical systems involved in the process of designing and testing. In order to measure the internal stray radiation in infrared imaging systems, a method based on radiometric calibration is proposed, and its rationality is verified by theoretical analysis and experiments. Firstly, the model of radiometric calibration without lens, in other words the detector for absorbing the radiation flux directly from the calibration reference source, is developed to show the influence of internal factors of the detector on the system output. Then it is compared with the results of calibration of the infrared system with a lens to obtain the system output results from the optical system, namely the internal stray radiation caused by the optical system. Finally, experiments are performed to prove the correctness of the theories proposed in this paper. The proposed method has some advantages, such as simple operation, low demand for the experimental conditions, and the capability of measuring the internal stray radiation accurately. It can be used to guide the stray radiation suppression in the process of infrared system designing, to verify the stray radiation analysis results, and to test whether the stray radiation level meets the practical requirements.