Many Micro-Electro-Mechanical Systems (MEMS) devices such as accelerators, gyroscopes, uncooled infrared sensors, etc., require vacuum packaging. The vacuum maintaining lifetime directly determines the vacuum packaging reliability. This research presented a quantitative analysis of the relationship between the leak rate and the vacuum maintaining lifetime, and demonstrated that the leak rate measurement plays an important role. This paper also explored the application limitations in vacuum packaging using a helium spectrometer leak tester to measure the leak rate because the measured leak rate was nonlinear with respect to the actual leak size. According to the fact that the damping coefficient changes with the pressure, a tuning fork crystal chip as a pressure sensor was used to monitor the pressure changes in the package cavity. The leak conductance was also calculated from the pressure tracking data to analyze the leak modes; the molecular flow model and gas desorption model were found to fit the measurement results of leak conductance.