The invention provides an extruding device and a method for double-mouth and thick-wall pipe blank extrusion manufacturing, and belongs to the technical field of thick-wall pipe extrusion manufacturing. The device comprises an upper ejector rod, a lower ejector rod, an extrusion cylinder and a floating extrusion cylinder cover; the device is mounted on an extruder; the outer surface of the extrusion cylinder is tightened and fixed by the floating extrusion cylinder cover; the floating extrusion cylinder cover is connected with an extrusion cylinder oil cylinder; the upper end of the upper ejector rod is connected with an extrusion shaft of the extruder through a fixed plate; and the lower end of the lower ejector rod is connected with an extruder lower platform through a fixed plate. In the use process of the device, the upper ejector rod downwards moves for extrusion at a certain speed; meanwhile, the oil cylinder drives the floating extrusion cylinder cover to downwards move at a certain speed to realize synchronous formation of upper and lower pipe mouths. Compared with a traditional forging process, the method has such characteristics as short manufacturing flow, high efficiency, excellent performance and low cost, and can be applied to high-performance forming production of high-performance thick-wall pipes and pipe fittings with pipe mouths for nuclear power and thermal power.