The invention discloses a low-temperature heat pipe heat exchanger with carbon dioxide as a working medium. A round separation device is arranged between an upper heat exchange shell segment and a middle heat insulation shell segment. The upper heat exchange shell segment and a lower heat exchange shell segment are connected through a channel. A piston capable of moving and a piston ring are arranged in the upper heat exchange shell segment. A wear-resisting coating and a porous structure medium layer are distributed on the inner wall of the upper heat exchange shell segment in the circumferential direction. A high-elasticity film layer for separating the piston and a cavity of the upper heat exchange shell segment is arranged on the lower portion of the piston. An ejector pin mechanism capable of opening the round separation device is arranged on the inner wall of the upper heat exchange shell segment. During work, outside cold fluid flows through the upper heat exchange shell segment, the temperature of carbon dioxide gas in the upper heat exchange shell segment is reduced, the carbon dioxide gas is directly changed into a solid state from a gas state, and then solid carbon dioxide absorbs heat in the lower heat exchange shell segment to be changed into the gas state. The gas-solid two-phase heat exchange principle is adopted, the carbon dioxide is adopted as the working medium, the ozone depletion potential (ODP) is zero, the global warming potential is quite low (GWP=1), and toxicity and corrosiveness are avoided.