Cesium iodide film has the high photoelectric conversion efficiency for ultraviolet light and X-ray, but it is prone to deliquesce in air. The preparation and protective effect of the anti-deliquescence ultra-thin protective film of cesium iodide film on micro-channel plate (MCP) were introduced. The coating depth and thickness of cesium iodide thin film photocathode MCP were tested by scanning electron microscope (SEM). Alumina was used as the protective film of cesium iodide thin film photocathode, and the alumina protective films with thickness of 2 nm, 5 nm and 10 nm was prepared respectively. After storing in the air for different times, there was no obvious deliquescence change on the surface of the cesium iodide thin film photocathode MCP. The gain is about 8 800, and the dark count rate is about 4.1 counts·s^?1·cm^?2. It is proved that the alumina can be used as the anti-deliquescence ultra-thin protective film of cesium iodide film on MCP.