The study on packaging materials of flexible organic light-emitting devices (FOLED) has become a hot spot of FOLED research. How to measure the transmission rates of water vapor, oxygen and other active gas through the encapsulation materials of FOLED is an important question of the FOLED packaging material research. A new method for measuring the gas transmission rates through packaging materials of FOLED by mass spectrum technology is provided in this paper. A mass spectrum measuring system is established for determine the permeability of the packaging materials. The principle of the measuring system is introduced. Permeation rates of water vapor, oxygen and carbon dioxide through PET plastic film, and of water vapor through ITO film and silver film were measured. The measurement results are credible by comparing the data with which other literature reported.