The invention discloses a liquid forging forming method for a welding-neck flange, a forming die and a forming device. The liquid forging forming method includes the following steps that firstly, molten metal is injected into a die cavity between an upper die and a lower die of the forming die; secondly, the upper die moves downwards relative to the lower die so as to compress the disc forming part of the die cavity, and an extrusion ring slides in a groove to compress the neck forming part of the die cavity; and thirdly, after a liquid-forged piece is cooled, the liquid-forged piece is taken out. According to the liquid forging forming method for the welding-neck flange, the forming die and the forming device, in the compression process of the die cavity, except for the main compression action of moving the upper die downwards relative to the lower die to compress the disc forming part, the extrusion ring can slide in the groove relative to the groove wall, and the additional compression action of the neck forming part is achieved; and in fact, pressure on the molten metal in the neck forming part is further enhanced through the extrusion ring, the neck portion, with compactness the same as that of the disc portion, of the liquid-forged piece is obtained, and the pressure applying effect of the neck portion of the welding-neck flange of the forming die is improved.