Transmissivity is a crucial parameter for an optical system, especially for an infrared imaging system, which is commonly used for detecting and tracking dim target. NETD (Noise Equivalent Temperature Difference) is an important indicator for quality of an infrared FPA system, and it is always related with the transmissivity of its optical system, which is dealing with energy transmission of the system. So it is necessary to measure the transmissivity when optical system is fabricated. However it is usually not easily available, when the structure of IR optics is complex in particular. In this article, a new reliable method for measuring the transmissivity of optical system for cooled infrared detector is introduced. The principle of measurement is based on Etendue conservation and luminance (radiance) conservation when light travels through an ideal optical element. We put a Lambertian blackbody source attached at the cold stop of IR detector of under testing system, and get the grey-scale value of axis point of detector. Afterwards we put on the optical system for the detection and calibrate the optical path for imaging system. Then we put the Lambertian blackbody source at the position of entrance pupil of optical system, and get the grey-scale value of axis point of detector again. With proper calculation with these two grey-scale value, we could get transmissivity of this optical system. A ray tracing simulation is made to show the result for supporting correctness of this approach. A high-aperture Lambertian blackbody source is required for the embodiment of this measurement.