The invention discloses an automatic paint spraying device for a round metal disk and a paint spraying method thereof. The automatic paint spraying device for the round metal disk comprises a spray gun bracket, a spray gun, a first servo motor, a second servo motor, a control box, a Z-shaped bracket, an X-axis guide track and a Y-axis guide track, wherein the spray gun and the first servo motor are arranged on the spray gun bracket. The automatic paint spraying device is suitable for spraying paint to the surface of metal with larger mass; during paint spraying, the round disk keeps still, a nozzle and a metal sensor perform X-Y axis plane movement above the round disk; and when the metal sensor detects that a metal object exists below the round disk, the nozzle is opened and can spray paint, and otherwise, the nozzle is closed. When the automatic paint spraying device for the round metal disk is used for spraying paint, the surface of the round disk with larger mass can be conveniently, automatically and uniformly painted, the consumption of energy and paint is reduced greatly, and the device is simple in structure, safe and reliable.