A novel large-caliber wellhead device for a high-pressure underground gas storage well is characterized by comprising a base (1), a compression cylinder (2), a gas feed and exhaust joint (3), a gas feed and exhaust valve rod (4), a blowdown valve rod (5), a blowdown valve rod pressing cap (6), a standby hole plug (7) and two O-shaped seal rings (8), wherein a casing joint (9) is sleeved outside and connected with an oil conduit (10) through a casing clasp (10a); the base (1) and the casing joint (9) are connected through a casing clasp (1a); the lower end part of the gas feed and exhaust joint (3) takes the shape of a tapered round platform (3a); the inner wall in the middle of the base (1) is a conical surface (1a); the two O-shaped seal rings (8) are arranged on the conical surface (1a) to achieve reliable sealing; the tapered round platform (3a) is pressed on the conical surface (1a) of the inner wall in the middle of the base (1) to achieve sealing; and the compression cylinder (2) is sleeved on the outer wall of the straight pipe of the gas feed and exhaust joint (3).