The invention relates to a method for expanding the use of a middle die device of a copper rotor die-casting die and belongs to the technical field of die-casting dies. The middle die device is the middle die device of a conventional copper rotor die-casting die. According to the method, different upper cushion blocks (2), supporting columns (6) and lower cushion blocks (8) are replaced to adjust the total length, wherein the upper cushion block (2) and the lower cushion block (8) are processed to be as long as each other, or to be composed of two or more cushion blocks; the cushion blocks are the same or different in thickness, and the thickness can be decided according to the change of the product length; each part is formed and fixed by use of the overall dimension or by use of a positioning pin. The method is simple and convenient to operate; the repeated machining of the middle die is avoided; the free change of different iron core lengths of the same product can be realized; the die machining cost can be greatly reduced; the inventory pressure is reduced.