The present application discloses a light sensor circuit, which comprises a photodiode and a capacitor unit. The cathode of the photodiode is controlled by a capacitive unit to maintain the same or close voltage level as the anode of the photodiode, which significantly reduces the effect of the dark current of the photodiode. Thus, the light sensor circuit can effectively maintain the performance and accuracy of an analog-to-digital converter applying the light sensor circuit. The circuit design difficulty and manufacturing cost are also significantly reduced.