The invention discloses a water-cooling welding clamp, a manufacturing method of the water-cooling welding clamp, a case clamp and a welding method. The water-cooling welding clamp comprises a positioning structure which is used for fixing and positioning bosses in the welding position of a case, and a heat dissipation structure which is used for heat dissipation and cooling during water-cooling welding of the bosses onto the case. A gas flow channel which is used for transmitting protective gas into a cavity defined by the bosses and the case is disposed in the positioning structure, and a water flow channel which is used for utilizing flowing cooling water to perform heat dissipation cooling on the welding part is disposed between the positioning structure and the heat dissipation structure and/or in the heat dissipation structure. By means of the water-cooling welding clamp, heat absorbed by the heat dissipation structure is constantly carried away through the cooling water in the water flow channel in the welding process, so that the heat dissipation property of the heat dissipation structure is improved, the heat dissipation structure can constantly absorbs the heat around a welding joint, and then severe deformation of the bosses and the case due to too high temperature is avoided.