Abstract: By discussing the effect of light source illumination on measuring modulated transfer function (MTF) of image intensifier, the light intensity through slit used in MTF testing system was analyzed. By adjusting the illumination on slits surface, measuring MTF in different illuminations and comparing it to the maximum output brigtness, it is concluded that the relationship between incident light illumination and MTF of the 3rd low-light-level image intensifier agrees with parabola distribution, there is a maximum value of MTF which restricted by the automatic brightness control (ABC) of image intensifier. The suitable incident light illumination can be specified by this method, which ensures enough signal to noise ratio in tested image intensifier and does not cause image intensifer to saturate.