Abstract: In order to ensure the personal safety and avoid the inconvenience of the high pressure gas cylinder and the emission of contaminated working gases, the gas detector has been changed from the gas-flow type into the enclosed type. The enclosed type gas detector can enlarge applications of the gas detector and improve competitiveness with the solid detector. The disadvantage of enclosed type gas detector is that the impure gases will accumulate in the cavity, which will affect the detector performance (gain, resolution and efficiency) and cause high voltage spark. In this paper, the helicoflex sealing was used to realize the dynamic high vacuum seal, and the aluminum-stainless steel composite plate was used to realize the welding of aluminum and stainless steel. The high temperature baking and high vacuumizing by vacuum pump group were used to realize the cavity purifying. Especially, the micro-arc oxidation was used for the purification of the vacuum chamber. The gas generation rate of the impurities in the enclosed cavity is reduced by 12.1% after micro-arc oxidation. The hydrogen content is reduced by 36%. The contents of water, carbon monoxide and carbon dioxide almost all fall in an order of magnitude. The content of the impurity gas within 1 a will accumulate to 0.006%. Within 2 months the content of water will accumulate to 0.000 5%, which can ensure the detector works steadily for 2 months without a high voltage spark. The enclosed cavity developed is suitable for the enclosed type gas detector of neutron spectrometer.