The invention discloses a friction stir welding method for a stirring pin and a shaft shoulder during differential speed rotation, relates to the friction stir welding method and solves the problems of high downward pressure and high forward resistance and various problems caused by overcoming high speed rotation of the stirring pin and high speed rotation of the shaft shoulder in the same direction or non-rotation of the shaft shoulder existing in conventional friction stir welding. The method comprises the following steps of: inserting the stirring pin of a stirring head into a part to be welded at a rotation speed of between 3,000 and 30,000 rpm, wherein the rotation speed of the shaft shoulder of the stirring head is less than that of the stirring pin and the rotation direction of the stirring head is reverse to that of the stirring pin; the rotation speed of the shaft shoulder is 10 to 1,000 rpm; the insertion speed of the stirring head is 1 to 5 millimeter/minute; the depth of the stirring pin inserted into a workpiece is 0 to 0.5mm less than the thickness of the workpiece; and the depth of the shaft shoulder inserted into the workpiece is 0 to 0.5mm; and moving the stirring head along the horizontal direction of a butting surface of a first welded workpiece and a second welded workpiece at a speed of between 100 and 5,000 millimeter/minute. The heat required by the welding does not depend on the friction between the shaft shoulder and the welded material, so high downward pressure is not needed to ensure the friction of the shaft shoulder part to produce heat and the downward pressure is reduced.