A method for testing air tightness includes connecting a testing chamber and a storage chamber, supplying negative pressure to the storage chamber, measuring the pressure in the storage or testing chamber to obtain a first pressure value, determining air tightness of the testing chamber according to the negative pressure and the first pressure value, stopping the negative pressure to the storage chamber, measuring the pressure in the storage chamber to obtain a second pressure value, measuring the pressure in the storage chamber after stopping the negative pressure to the storage chamber to obtain a third pressure value, and determining air tightness of the testing chamber according to the second and third pressure values. The device includes testing and storage chambers, a negative pressure generator, and a pressure gauge connected to the storage chamber, which is connected to the testing chamber. The negative pressure generator is connected to the storage chamber.