An auto light intensity adjustment system is provided. The auto light intensity adjustment system includes a light sensing unit, a charge/discharge circuit, a comparator unit and a light source module. The light sensing unit outputs a sensing signal according to an ambient light intensity detected thereby. The charge/discharge circuit is coupled to the light sensing unit for receiving the sensing signal, and generating a voltage signal with regular variation periods according to the sensing signal. The comparator unit compares the voltage signal to a threshold value signal, and outputs a result signal accordingly. The light source module modifies a light intensity according to the result signal. The invention automatically controls the light intensity according to the ambient light intensity.