A polishing mechanism for treating a vehicle bumper incorporating with a multi-shaft robot is disclosed. A polishing wheel is rotationally attached to an end of the robot. A rotational positioning device is provided for positioning the bumper in a position within a working range of the robot. The positioning device includes a pair of racks, a main shaft rotationally arranged between the racks and capable of being positioned to a desired angle, a carrying rack arranged on the main shaft, and a plurality of holding devices arranged on the carrying rack for fixedly holding the bumper in position. A detecting device is provided for checking the worn-out of the polishing wheel for adjusting an optimum distance between the polishing wheel and the bumper. The assembly further includes a piloting gauge attached to the robot for determining an appropriate distance between the polishing wheel and the bumper.