In order to keep the detector chip stable at low temperature,the vacuum micro Dewar needs a high vacuum degree to reduce the heat exchange between Dewar and the outside world.The vacuum degree of different Dewar failure is different.In this paper,the limit vacuum degree of the above two Dewar structures is determined by static thermal load test under different vacuum degrees for 640×512 μm and 320×256 Dewar components.By comparing and analyzing the vacuum failure mechanism and experimental data of Dewar assembly,the measures to improve the vacuum life of Dewar assembly are put forward.