To support the correct design of cryogenic optical systems, a measuring system for the detectivity of infrared detectors in low temperature background was established and applied to some infrared detectors for testing their detection characteristics. First, the theoretical analysis for noise and response characteristics of an infrared acquisition system was introduced, and the relationship between the design of a low temperature optical system and the detectivity of infrared detector in the low temperature background was established. Then, a measurement system of detectivity in the low temperature background based on thermal vacuum enviroment was proposed, and experimental research on some infrared detectors in the low temperature background was accomplished. Finally, the variation regularities of limiting integration time and detectivity in low temperature compared with those in normal temperature case were discussed as well. Experimental results indicate that both of the integration time and detecitivity in the low temperature background are 20 times that in normal temperature background. The regularity derived can satisfy the requirments of system index design of low temperature optical systems.