The invention relates to a water welding machine which comprises a machine box, an electrolytic groove, an oxygen separating device, a hydrogen separating device and a control box, wherein the bottom of the machine box is provided with the electrolytic groove; the oxygen separating device and the hydrogen separating device are fixed above the electrolytic groove; a gas generated by electrolysis enters the oxygen separating device and the hydrogen separating device to separate hydrogen and oxygen; an electrolytic pipe extends from one side of the electrolytic groove and is communicated with the electrolytic groove; a radiating fin is arranged on the electrolytic pipe; and an electrolytic electrode for electrolysis is arranged in the electrolytic pipe. The side part of the electrolytic groove is additionally provided with the electrolytic pipe, the electrolytic pipe not only increases the electrolytic space, but also can greatly improve the electrolytic efficiency; the sufficient gas isprovided for the water welding machine so that the water welding machine have sufficient firepower to weld; and the internal space of the machine box can be fully utilized.