The quality of Al2O3 film evaporated on the input side of micro-channel plate is analyzed with electron microscope and mass spectrometer, and the influence of the film on the device performance is investigated to solve the problem of lifetime of 3rd generation low-light-level (LLL) tube. The effect of Al2O3 film on the imaging quality of 3rd generation LLL imaging devices is discussed. The research result shows that Al2O3 film affects the imaging quality of the LLL tube seriously, makes images dim and reduces the noise-to-signal ratio though it can effectively prevent the ion feedback. Another effective measure for solving the problem of device lifetime essentially is proposed, in which the vacuum isolation is implemented between photoelectric cathode and display screen to avoid the bombardment of ion feedback to the photoelectric cathode.