The invention discloses a mark period level light intensity accumulation based method for determining and searching an alignment mark and an alignment system. The method is characterized by comprising the steps of: acquiring light intensity of first level light of each period of a multiple-period mark, accumulating the light intensity to obtain a light intensity signal, and acquiring a position of an alignment mark through the light intensity signal; according to a preset experience threshold of each period, screening a burr point with abnormal light intensity value in the light intensity signal; conducting a minimum threshold inspection on the light intensity signal; if the light intensity signal does not meet the minimum threshold, then conducting position correction; if the light intensity signal meets the minimum threshold, then superposing first level light intensity of each period; after superposition of the light intensity, detecting whether slopes of a highest peak point, a left peak point and a right peak point meet the requirements; if it does not, then conducting position correction; if it does, then using the point as a mark position obtained by superposition of light intensity; and then calculating an absolute value of the mark position with an issued desired mark position to determine whether an alignment mark is successfully searched. The invention also discloses an alignment system, and the alignment system employs the mark period level light intensity accumulation based method.