Abstract-CsI is a great photoelectric emission material in the field of photon imaging detectors owing to its high quantum efficiency in the ultraviolet range. But quantum efficiency depends on the electron emission mode deeply. With specific structure of photocathode, the photoelectron can operate in both semitransparent mode and reflective mode. So some experiments were done to verify which mode is better and how to get it. Theory and experiment show that the quantum efficiency in reflective mode is higher than Quantum Efficiency in semitransparent mode. For a certain specifications of the vacuum tube, the parameter of reflective mode fabrication was obtained. Particularly in double proximity focusing image intensifier tube system, CsI photocathode can be deposited on micro channel plate(MCP), and avoid applying high voltage between the incident window and MCP, which is micro distance. Above all, this enable photoelectric emission to operate in the reflective mode, thus higher quantum efficiency can be obtained. The thickness of CsI film influence the quantum efficiency and it depend on the assembly structure of MCP. Some theory analysis and experiment proved it in this paper.