The invention discloses a double-layer drainage device used for underwater laser cladding and underwater laser-arc composite welding, and relates to the field of underwater laser cladding and laser-arc composite welding. The double-layer drainage device includes an upper end cover, an inner layer drainage tube, an outer layer drainage tube and a drainage sealing gasket. The upper end cover is provided with a welding opening. A glass plate is arranged on the welding opening. An outer drainage cavity is defined by the outer layer drainage tube, the inner layer drainage tube and the upper end cover. An inner drainage cavity is defined by the inner layer drainage tube and the upper end cover. The upper end cover is provided with at least three inner drainage cavity air inlets in an annular arraying mode. At least three outer drainage cavity air inlets are evenly distributed at the upper part of the outer layer drainage tube in the circumferential direction. The drainage sealing gasket is arranged at the lower end of the outer layer drainage tube and is provided with an annular flexible rubber skirt bushing. The lower part of the outer layer drainage tube is provided with a welding auxiliary joint. The double-layer drainage device is ingenious in structure, convenient to use and high in adaptability, underwater laser cladding or underwater laser-arc composite welding work can be completed, and the working efficiency and quality of underwater laser welding work are improved.