The invention discloses a variable step size monitoring AGC automatic control method for rolled piece thickness control. Through the method, a thickness adjustment coefficient KM is automatically indexed according to the difference of a measured actual thickness and a target thickness; different temperature adjustment coefficients Kp are adopted according to the variation of a finished rolling actual temperature and a set temperature; the product of the thickness adjustment coefficient KM multiplied by the temperature adjustment coefficient Kp serves as a final adjustment coefficient K; the AGC adjustment step size of the final n racks of a roll train is adjusted according to the final adjustment coefficient K, the thickness of a rolled piece can be controlled accurately, and the qualifiedrate of the thickness within the tolerance range of +/- 30 microns can reach 100%. Over-adjustment and adjustment quantity insufficiency cannot occur.