A light full-sealed underwater automatic welding wire feeder comprises a wire feeder box, a wire reel mechanism, a speed reduction transmission mechanism, a gear transmission mechanism, a wire pressing mechanism and a sealing motor with a sealed waterproof function, wherein the wire feeder box is provided with multiple interfaces, and the wire reel mechanism, the speed reduction transmission mechanism, the gear transmission mechanism, the wire pressing mechanism and the sealing motor are sealed and insulated in the wire feeder box. The interfaces on the wire feeder box include the gas way interface, the power cable interface and the welding gun interface. The inside-box gas pressure balanced with the outside-box water pressure is formed in the wire feeder box through protection gas injected into the wire feeder box through the gas way interface. The sealing motor, the speed reduction transmission mechanism and the gear transmission mechanism are sequentially connected. A pressure passageway is formed between the installed wire pressing mechanism and the installed gear transmission mechanism. A welding wire wound on the wire reel mechanism is pressed when passing through the pressure passageway and is conveyed to the welding gun interface of the wire feeder box. The light full-sealed underwater automatic welding wire feeder has the beneficial effects that the weight is small, the sealing performance is good, and the possibility that the device fails when working under water can be greatly reduced. The invention belongs to the technical field of underwater welding.