Vacuum technology is extensively used in space science, nuclear energy, surface science, materials science, electric vacuum industry, microelectronics, semiconductor, metallurgy, and so on. More and more fields need ultra high vacuum (UHV) or extreme high vacuum (XHV) for scientific research and production. Both the rigorous vacuum processing technics and highly developed sophisticated vacuum technology are the necessities for the acquisition and conservation of UHV or XHV ambience. In this paper, a vacuum decline was analyzed and the UHV system was recovered. Using the residual gas analyzer to measure the partial pressure of residual gas and identify the type of gas, the outgassing source was found out by comparing the measure results. The UHV system was recovered back to 10-9Pa through treatment. The residual gas analyzing method can effectively identify the type of gas about outgassing, narrow the scope of outgassing subassembly, shorten the vacuum system recovery time, and improve work efficiency. The method of determining the outgassing source by residual gas analyzer can be applied in the fields of UHV and XHV, and it has a certain reference significance for further improving the system vacuum degree.