The invention relates to an automatic polishing device, in particular to an automatic polishing device for a motor. An outer surface flexibility polishing device of a motor rotor comprises a big support, an automatic fixture and a flexibility polishing mechanism, the automatic fixture is used for installing the motor rotor, the flexibility polishing mechanism is used for polishing the motor rotor, the automatic fixture and the flexibility polishing mechanism are fixedly connected to the big support, the flexibility polishing mechanism is arranged at the upper part of the automatic fixture, and the motor rotor is connected onto the automatic fixture. By the outer surface flexibility polishing device, the outer surface of the motor rotor is polished, the outer surface flexibility polishing device of the motor rotor, disclosed by the invention is high in manufacturability, the motor rotor can be automatically clamped and driven to rotate at a certain speed, a hemp wheel has functions of rotating at a high speed and doing axial movement along the motor rotor, and the polishing quality and efficiency are greatly improved.