Abstract: The paper reviews the ion barrier adopted in 3rd generation image intensifier, which has dual effects on the GaAs photocathode, protecting it from being damaged by ion feedback, at the same time degrading the quantum efficiency. The origin of pernicious gases and the formation mechanism of ion feedback are explored, then some restraint techniques are introduced, including the latest developed GaAs image intensifiers with a highperformance MCP as its feature, which made the film significantly thinner or even removed due to the large reduction of pernicious gases. The latest bulk conductive MCP and Si MCP with excellent advantage on restraining ion feedback are exhibited. The development of longlife filmless GaAs image intensifier remains a great challenge. Further progress in MCP is needed to restrain ion feedback, and the GaAs photocathode’s resistance to ionized and native gases still needs enhancing.