The invention discloses a mounting method for a nuclear power plant generator stator portion sleeve. The method comprises steps that a bearing and a rotor are mounted on an end cover, two reference points are selected from the end cover, and shortest distances 1a and 1b from the two reference points to an outer arc surface of the rotor are respectively measured; the bearing is turned out, and shortest distances 1a' and 1b' from the two reference points to the outer surface of the rotor are measured; the rotor position is adjusted, 1a' is made to be equal to 1a, 1b' is made to be equal to 1b, and the rotor position is fixed; the stator portion sleeve is mounted. Compared with the prior art, repeated tile turning is not needed, manpower and material cost is saved, and a standard construction level and construction efficiency are improved. The invention further discloses a device used for measuring a distance between the nuclear power plant generator rotor and the end cover and a device used for supporting the nuclear power plant generator rotor.