For film gas permeability test device based on differential pressure, when the performance of pressure and temperature detector is given and the test temperature is relatively stable, the test result is decided by seal performance of test device. To improve the seal performance and test performance of test device, the model of three layers sealed structure was presented and the corresponding mathematic model was established. The feature of pressure change caused by leakage and its simulation result were given. The purpose was to provide theoretical basis for practical applications of film gas permeability test device with three layers sealed structure.