The invention discloses an ultra high pressure hydraulic cylinder and belongs to the technical field of hydraulic equipment. According to the ultra high pressure hydraulic cylinder, motive seals are arranged on the inner wall and the outer wall of a main plunger respectively, so that a fluid chamber is formed by the space defined by a main cylinder body, the main plunger and a return cylinder body, and the bottom of the main plunger is connected with an indenter; a motive seal is arranged on the outer wall of the bottom of the return cylinder body, so that the space between the outer wall of the return cylinder body and the inner wall of the main plunger is designed to be a return cylinder; a tubular welding ring is arranged in the return cylinder body, the two ends of the welding ring are sealed with the return cylinder body respectively, so that the space between the outer wall of the welding ring and the return cylinder body is designed to be a liquid return chamber, and a liquid hole communicating the liquid return chamber with the bottom of the return cylinder is arranged on the wall of the return cylinder body; a displacement monitoring mechanism is arranged on a probe base and/or the inner wall of the welding ring. According to the ultra high pressure hydraulic cylinder, the main plunger is used as the main plunger of a main hydraulic cylinder, and the inner wall of the main plunger is used as a cylinder body of the return cylinder, so that the components of the hydraulic cylinder are greatly simplified, and the weight of the hydraulic cylinder is reduced.