Abstract: In order to resolve the pollution effects of ion barrier Al2O3 film on GaAs photocathode sensitivity of third-generation low-light-level(LLL) tube ,we used the quadrupole mass spectrometer to analyze the gas composition released from the ultra-high vacuum chamber ,the microchannel plate(MCP) without film and the MCP with Al2O3 film under electron bombardment. The results show that the MCP with Al2O3 film releases the residual gases of C, CO, CO2, NO, H2O2 and CXHY, which come from the quality pollution during the preparation process of Al2O3 film .After improving the filming technology, we obtained the Al2O3 film with outgasing amount less than 210-9 Pa and without CXHY.