The invention provides a nuclear main pump rotor centering tool, which comprises a supporting part, a rotating part and a driving part. The supporting part comprises a mounting bracket and a speed reducer bracket fixed above the mounting bracket. The driving part comprises a motor and a speed reducer. The rotating part comprises a first key, a baffle, a coupler, a second key and a tool shaft. Themotor, the speed reducer, the first key, the baffle, the coupler, the second key and the tool shaft are coaxially connected from top to bottom. The speed reducer is fixed to the speed reducer bracket.The tool shaft penetrates through the mounting bracket to be coaxially connected with a rotor to be centered, and a bearing and a bearing sleeve are arranged at the position, penetrating through themounting bracket, of the tool shaft. By using the nuclear main pump rotor centering tool, manpower can be fully liberated, and centering inspection of a nuclear main pump motor rotor can be completedmore quickly, better and more safely.