Vacuum sizing machine is to put wax strings into the hanging rack of the sizing chamber, close the door of the room, empty the air in the sizing chamber, and automatically drop the hanging rack into the sizing barrel. At this time, because there is no air residue in the wax itself, the sizing material can completely cover the wax, leaving no dead corners, avoiding the shortcomings of manual sizing that some corners can not be touched negligently. Wax strings stay in the slurry for a period of time, automatically lifted, vacuum relief, kettle door open, into floating sand or sanding operations.