The invention provides a method for milling a slot by an intelligent card slot milling machine. The method for milling the slot comprises the following steps of: (1) arranging two milling cutters with different diameters at two stations of the slot milling machine which run synchronously and differentiating a rough cutter from a fine cutter; (2) cutting in a vertical Z direction by using the finecutter, controlling the feeding of the fine cutter along a square path in the X direction and the Y direction of a plane and machining the profile of a card according to requirements; (3) adjusting feed in the X direction and the Y direction according to the difference of the diameters of the two milling cutters, ensuring that the rough cutter also machines the profile of the step (2), simultaneously cutting with the two cutters, and feeding along the square path according to the set feed so as to machine the profile of another card by the rough cutter according to size requirement of a square recess of the SIM card, wherein a part of non-milled material in the square recess is milled off by the fine cutter in the process; (4) simultaneously cutting off the remaining materials in the square recess of the two cards by the two milling cutters to ensure the same size of the profiles of the square recesses of the two cards respectively; and (5) simultaneously cutting to machine circular recesses by using the two cutters. The method is easy and convenient to operate and high practicability.