Abstract: Micro-channel plate (MCP) with ion barrier film is one of the key components of Low Light Level(LLL)image intensifier with negative electron affinity, and working state of filmed MCP has a serious influence on it, so MCP best working voltage should be determined by testing filmed and un-filmed MCP gain in different voltage of MCP voltage and cathode voltage. ①For LLL image intensifier with negative electron affinity with un-filmed MCP, optimum operating voltage are: if cathode voltage exceeds a certain value V_c1 with MCP gain almost unchanged, this explains that cathode voltage V_c1 at this time is the best operating voltage; if MCP voltage is of a specific value V_m1 (cathod voltage is any value higher than V_c1) with low MCP gain, and MCP gain turns higher enough when > 20 000 MCP voltage becomes (V_m1+100V), thus this value (V_m1+100V) is the optimum MCP voltage; ②For the same material with filmed MCP, operating voltage are: cathode voltage V_c=algebraic sum of un-filmed MCP cathode voltage (V_c1) and threshold voltage, and MCP voltage is V_m > (V_m1+100V), whose specific value should be determined according to linear region of filmed MCP gain value. This study has an important significance to the determination of best working voltage and performance improvement of filmed MCP on NEA photocathode image intensifier.