The main factor affecting the vacuum life of the infrared detector Dewar is the internal material outgassing. The Langmuir adsorption model outgassing equation is used to calculate the vacuum lifetime due to the outgassing mechanism and the long-term degassing procedure of the Dewar. An innovative method of changing the temperature at the end of the degassing procedure was proposed to obtain the outgassing activation energy. The outgassing rate was measured at different temperatures while simultaneously removing the influence of coverage on the outgassing rate. The outgassing rates at storage temperature and the rule of outgassing rate changing over time were deduced. The difference among the three Dewar outgassing activation energies obtained in different conditions was 8.8%. The heat load of the Dewar was tracked for two years to verify the method. The estimated error of Dewar's vacuum lifetime was 7.2%. It is a non-destructive testing estimation method for the vacuum life of small and diverse Dewar.