The utility model relates to a wellhead ascension monitoring device for offshore heavy oil thermal recovery. The wellhead ascension monitoring device for offshore heavy oil thermal recovery comprises a barrel-shaped body, a plurality of seal rings, a tube-shaped tubing telescopic compensator, a casing, a spring device, a stress sensor and a computer display system, 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; the tube-shaped tubing telescopic compensator penetrates through the body in the axial direction; the tubing telescopic compensator is sleeved with the casing; the spring device is connected with the upper end of the casing through a spring base; the stress sensor is installed at the upper end of the spring device; the computer display system is electrically connected with the stress sensor. 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 formed 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 monitoring 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 monitoring device, in a heavy oil thermal recovery process, the situation that because the casing is extended, a Christmas tree ascends is avoided, and it is guaranteed that oil production operation proceeds normally; besides, the extension length of the casing can be effectively monitored in time, guidance is provided for operation, and the operation risk is reduced.