To study the influence of laser radiation on the performance of an image intensifier, the irradiation experiment of CW laser on the image intensifier was carried out and the effect of laser irradiation on its gain characteristics was analyzed. Experimental results show that no pixel crosstalk phenomenon is found as the increase of the laser power until the light illumination arrives 8×104 times of the illumination for point saturation, meaning that the gain of the image intensifier is saturated and the output light intensity of image intensifier will not increase indefinitely. The equivalent circuit model of the microchannel plate of the image intensifier was established and its gain characteristic was analyzed. The maximum current for linear gain, got from this model, was approximately 1.64×10-10 A, which was very close to experimental data, indicating that the charge extracted from the microchannel wall cannot be replenished in time is the main reason of gain saturation.