Abstract: Ion barrier film of Micro-channel Plate(MCP) is one of unique features of low-light-level(LLL)image intensifier with negative electron affinity. Its main function is to effectively prevent positive ions from bombarding cathode to improve working life of the device. Thickness of ionizing membrane of MCP determines size of electron permeation and ion blocking ability, and electron permeability directly affects contrast and signal to noise ratio(SNR) of the image. According to threshold voltage definition and test principle of ion barrier film of MCP, optimal thickness of ion barrier film is determined through lots of experiments. Results show that threshold voltage of ion barrier film increased linearly with increase of film thickness, linear relationship Y=3.98X+50 is proportional between two sides. Threshold voltage of ion barrier film decreased linearly with the increase of MCP operating voltage, showing an inverse proportion. This study is of great significance to improve performance of MCP in LLL image intensifier with negative electron affinity photonic cathode photoluminescence enhancers.