As a link between the earth's surface and its deep interior, the subduction zone is the primary tool to bring surface carbon into the deep earth and the principal place to exchange surface and in-depth materials. Subduction can bring surface carbon into the deep earth in organic carbon or inorganic carbonate minerals and then return to the surface system through volcanism or degassing. The deep carbon cycle in the subduction zone controls the surface carbon flux, which is of great significance for studying global climate change and the earth's habitability. This paper discusses the mechanism of carbonation and decarbonation in the subduction zone. The decarbonation mechanism includes decarbonizing metamorphic reaction, carbon dissolution, and melting decarbonizing. Carbon bearing fluids released from the subducting plate may not always migrate back to the surface. Some of them will react with the surrounding rocks (carbonation) to form other solid carbon phases (carbonate, graphite, or diamond), sequestrated in the subducting plate and overlying mantle wedge. This process will affect the carbon fluxes in different reservoirs and should be considered when the carbon flux released from the subduction zone is calculated.