The invention relates to a mandrel device used for a pipe bender during the cold bending processing of metal pipes. A mandrel device used for a pipe bender comprises a mandrel pull rod joint, a mandrel pull rod and a mandrel body; the two ends of the mandrel pull rod are respectively connected with the mandrel pull rod joint and the mandrel body thread; the device also comprises an anti-rotating hollow sleeve of the mandrel body; one end of the anti-rotating hollow sleeve of the mandrel body is matched and connected with the clearance of the mandrel body; one end of the mandrel pull rod passes through the anti-rotating hollow sleeve of the mandrel body and is connected with the mandrel body thread; the anti-rotating hollow sleeve of the mandrel body and the mandrel pull rod are oppositelyand fixedly connected through a fastener. According to the technical proposal of the invention, the anti-rotating hollow sleeve is additionally arranged at the jointed place of the mandrel body and the mandrel pull rod when the pipe bender fills the banding pipe in the inner cavity of the pipe by the metal mandrel; therefore, the mandrel body is effectively prevented from rotating; the security of a pipe banding die and a pipe banding machine are ensured; and the manufacture quality of the pipes is improved.