The invention relates to a generator pumping through rotor centering method. The method comprises steps that a, a center of a stator bore is determined, a centering line is marked on the ground or an exciter platform in an orthographic projection mode according to a center line of the stator bore, and the centering line is on the pumping through stroke of a rotor; b, a centering rod is fixed on a rotor excitation end, and the centering rod is on a vertical plane passing through the center line of the rotor; and c, in a rotor pumping through process, according to the relative position of the centering rod and the centering line, the direction of the rotor is adjusted, the rotor is observed from a generator excitation side to a steam side, if the centering rod is at a right side of the centering line, the rotor is pushed leftwards slowly, if the centering rod is at a left side of the centering line, the rotor is pushed rightwards slowly, and the centering rod is guaranteed to just point to the centering line when the rotor moves forwards or backwards. The method is advantaged in that a problem of stator-rotor bumping risks caused by supporting the rotor carried out by excitation side personnel and frequent communication among workers in the stator bore in the rotor pumping through process can be avoided, and work efficiency is improved.