This experiment employed mass-spectro-meter comparison method to measure the leak coefficient of ultra-thin beryllium film（d=18μm） of X-ray lithography beam line of Bejing Electron-Positron Colliding Device at a specially designed equipment. To enhance the sensitivity of leak detecting

preset leak detecting method was also introduced. The calculation results show that the leak efficiency is about 10

-5

Pa· m

3

/s. It is consistent with the leak efficiency data of the imported Berylliurn film given by the test sheet.