The utility model relates to a compensation device preventing wellhead ascension during offshore heavy oil thermal recovery. The compensation device comprises a barrel-shaped body, a plurality of seal rings, a tube-shaped tubing telescopic compensator and a casing, wherein the upper end of the barrel-shaped body is sealed, and the lower end of the barrel-shaped body is open; the seal rings are embedded in the body at intervals from top to bottom; the tubing telescopic compensator penetrates through the upper end of the body in the axial direction and then extends out of the body; the tube-shaped tubing telescopic compensator is sleeved with the casing. An annular reserved space exists between the tubing telescopic compensator and the body. The outer wall of the casing makes contact with the seal rings in the body, and a gap is reserved between the inner wall of the casing and the tubing telescopic compensator. The upper end and the lower end of the body are respectively provided with a flange structure, and the compensation device is fixed between a tubing head and a casing head through the flange structures at the upper end and the lower end of the body. According to the compensation device, in a heavy oil thermal recovery process, when multiple thermal fluids are injected in from the wellhead, the casing expands by being heated and ascends upwards along the reserved space of the body in the vertical direction, thermal stress is effectively released, and therefore the situation that because the casing is extended, a Christmas tree ascends is avoided, potential safety hazards are eliminated, and it is guaranteed that oil production proceeds normally.