The noise equivalent temperature difference (NETD) is an important parameter to characterize the sensitivity of infrared FPA detectors. In addition to the detectors, they are closely related to the application and test conditions. Starting from the NETD test method and test process, the NETD theoretical calculation formula for the photovoltaic infrared FPA detector was deduced, and the main factors affecting the NETD of the detectors were analyzed. The background-limit NETD of a typical infrared FPA detector under different integration times, storable charges, F numbers, and half-well fill output conditions was calculated and theoretically analyzed. Under a fixed background temperature, NETD with different temperature differences can be theoretically calculated from a certain temperature difference NETD. By selecting a typical detector for verification, a group of NETDs calculated from the normal NETD test results were found to be in good agreement with the test results under the corresponding temperature difference. The NETD calculation result for the 1 K temperature difference is significantly close to the actual situation, which can provide a reference for the application of thermal imager systems.