Abstract: The charge measurement process of CCD in normal work state has been analyzed. It is indicated that the mode of charge injecting to the measurement capacitor is the periodic narrow pulse that has a low fill factor when the CCD is normal. After the charge signal is measured, the feed-through level is restored under the effective resetting action.According to the condition under which the excessive saturation state of CCD happens and potential characteristics in the signal charge transfer channel of CCD,we put forward the physical mechanism of the CCD excessive saturation effect. That is, an optoelectronic current source is formed by laser in the irridiated spot of the CCD. The charge generated by laser crams the potential well of the CCD and then makes the CCD become a current device from a