Aiming at the requirements of low-temperature measurement of spacecraft in thermal vacuum or thermal balance test, on the basis of the principle of infrared thermal measurement, the factors influencing the cool measurement of the infrared system were detailedly calculated and analyzed, from the targeted radiation, background radiation and radiation sensitivity of the detector infrared by the infrared system. It was found that the stray radiation of the optical system was the key link to restrict the cool measurement capability of the current hot test. On this result, a set of infrared thermal imaging system was designed, which could be used directly in the vacuum thermal test environment. In the optical system, the temperature could be cooled into -60°C. The blackbody was used to radiate and calibrate the system. As the experimental result showed, the system could be used to realize the infrared radiation measurement of the 150K cool target, which filled the blank of the technical field in china.