An assembly method and fixture for assembling a supporting disk of a motor rotor according to the invention is based on an "adjusting, and fixing" concept. A functional axis fixture and an adjusting axis fixture, which can match with each other for holding the supporting disk and the rotor, are first prepared. The functional axis of the supporting disk is aligned with the geometric axis of the functional axis fixture, and the geometric axis of the rotor is aligned with the geometric axis of the adjusting axis fixture. By engaging the functional axis fixture and the adjusting axis fixture, the functional axis of the supporting disk is aligned with the geometric axis of the rotor. Therefore, the bias and concentricity can be precisely controlled and improved.