The invention relates to a method for changing die shape when overall dimension of a forming die or sample die changes. The method comprises the following steps: cutting a die body into two die bodies on a working face of the die according to the die shape, and then adding a die supplement which has the same material as the die bodies and the corresponding shape at the cutting place of the die, or removing the excess die body at the cutting place, connecting and fixing the die supplement and the two cutting die bodies of the die with locating pins and fitting bolts on a non-working face, and then remachining the die. In the invention, a method of increasing or decreasing the die bodies on the working face of the die is employed to cause the die to be applied to the forming die and modification of plane and automobile components at research and development stage of planes and automobiles, thus decreasing quantity of new dies.