Large area array infrared detectors are the core components in infrared remote sensing instruments. Most of them are composed of spliced small-scale area array detectors and are integrated on Dewar cold platforms to form Dewar assemblies. The stress state of a detector after being integrated on the Dewar cold platform is a key factor affecting the performance and lifetime of the chip. The thermal output of a strain gage when the detector is both in the free state and in the Dewar is tested at low temperature. The difference between the two values is used to characterize the extra strain after the detector is integrated with the Dewar cold platform. Taking a 2000×512 detector assembly as an example, the test verification and analysis are carried out. The result shows that this method is feasible.