The invention belongs to the technical field of welding technologies in material processing engineering, and relates to a single-pass multilayer welding device and a method for a medium plate, in particular to a single-pass multilayer GMAW (Gas Metal Arc Welding) method and a device for a 30-45mm thick medium plate. The device comprises a welding gun, a swing structure and a control system, wherein a welding joint groove is in a V-shaped belt backing plate groove form; an angle is 5-8 degrees; the gap width is 8-15mm; welding shielding gas adopts mixed gas of carbon dioxide and argon; a gas flow is 20-25L/min; and a metal transfer form during a welding course is spray transfer. The device and the method have the advantages that the welding efficiency of the medium plate can be improved effectively by the single-pass multilayer welding method, a side wall fusion effect can be improved by controlling arc swing, and the welding quality can be improved.