The invention discloses a working method of a vacuum painting device for a cylinder. The working method comprises the following steps: the cylinder is dip in liquid water; a variable frequency motor drives the cylinder to rotate to clean the surface; the current supply is stopped, two ends of an upper adjusting piece are positioned at the uppermost end of a first rail, the various frequency motor and a vacuumizing device are started, and the liquid water is heated through a heating resistor; paint is sprayed out from a nozzle to act on the surface of the rotating cylinder; the current supply to the variable frequency motor is stopped, and two ends of the upper adjusting piece downwards move along the first rail; the current is supplied to the variable frequency motor again, so that the paint sprayed out from the nozzle is uniformly sprayed on the peripheral surface of the cylinder; and the steps (5) and (6) are repeated until the surface of the cylinder is painted. The liquid water is boiled to generate steam in a working cavity; and as the paint is not dissolved in the water, more than 80% of the sprayed paint can cover the surface of the cylinder, the utilization rate of the paint is largely increased, and the waste is reduced.