Abstract: For a high power microwave source composed of ceramic vacuum interface, velvet cathode and stainless steel shell, which is sealed with knife-edge oxygen free copper gasket seals, the outgassing rates of the major materials and pumping down curves are simulated by the vacuum design software VacTran. The adsorption behavior of non-evaporable getter (NEG) is simulated by using a simple model in the high vacuum maintenance process. Pumping down times to 10-4 Pa in normal temperature and baked state are compared in experiments. NEG is adopted as inspiration pump when the vacuum reaches to the work pressure. The experimental results show that the pressure is still kept at 210-4 Pa after 30-day high vacuum maintenance. The accelerated life test of the high power microwave tube is done under the temperature of 80 ℃ for more than 140 hours, the pressure in the high power microwave tube is still lower than 110-2 Pa. Accordingly, the vacuum holding life lower than 110-2 Pa at room temperature is estimated more than one year.