Dewar vacuum failure is usually caused by IR FPA device malfunction, and the evaluation hereby used is testified as a valid method in discussing and analyzing such failure in accordance with Low Density Gas Theory of Vacuum Physics, The IR FPA Dewar device cooling time is regarded as the most direct and effective method in confirming such failure. Notice that the maximum heat loading variation between the maximum reactive power of IR FPA Dewar device and the rated capacity of the cooler should be in correspondence with its use-defined maximum cooling time. And Dewar vacuum life is considered unacceptable if the heat load variation being measured by LN Evaporation Principle is under such condition that the variation is within its specified time in the case it reaches expanded uncertainty(k=3). The value tested should be 30 mW. Within the customized vacuum service life of specified 15 years，the Dewar leakage and gas rate required must be less than 5×10^-15 Pa?m³/s. As the failure of Dewar vacuum occurred, and simultaneously, the gas pressure must be greater than 1×10^-2 Pa.