The invention discloses a forming device of an ultrahigh-pressure static liquid extrusion magnesium alloy pipe and a forming process thereof. The forming device totally adopts a horizontal structure, and comprises a supercharging cylinder, an extruding cylinder, a first pressing medium exchange disc, a second pressing medium exchange disc, a third pressing medium exchange disc, an extruding mold, a first heating sleeve, a second heating sleeve and a high-pressure hydraulic pump with an extruding shaft. The supercharging cylinder, the extruding cylinder, the pressing medium exchange discs, the mold and the high-pressure hydraulic pump are all fixed between a movable beam and a fixed beam; and the movable beam and the fixed beam are connected through a tension column. The supercharging cylinder and the extruding cylinder are arranged in parallel, and are positioned in the same one horizontal plane. The extruding shaft is positioned in the supercharging cylinder; a blank is positioned in the extruding cylinder; the travel of the extruding cylinder and the size of the blank both can be increased; the magnesium alloy pipe with a longer length can be obtained by static liquid extrusion machining through the forming process on the forming device; and the production efficiency can be improved.