Shellfish culture as a part of carbon sink fishery is doubtful. In order to explain the carbon sink function and identify the carbon source of shellfish, the carbon sources of three culture-bivalve were analyzed by using stable carbon isotope technique. The results showed that:the δ¹³C of the three culture-bivalve shells ranged from -6.9‰ to -6.3‰, the δ¹³C of soft tissue ranged from -23.2‰ to -21.9‰, and the enrichment degree of δ¹³C for shell and soft tissue was negatively correlated. The main carbon source of three culture-bivalve shells was dissolved inorganic carbon (DIC), followed by dissolved organic carbon (DOC), and the total contribution rate was above 86%. The major carbon source of soft tissue in three culture-bivalve was suspended solids, followed by sediments, with the total contribution rate of more than 94%. The shell and soft tissue of culture-bivalve showed extremely significant different selection in carbon source. Sediment was not the main source of carbon in culture-bivalve shells, most of the carbon converted into shells comes from the seawater DIC, which reduced the concentration of DIC in seawater, and promoted the CO₂ in the atmosphere transfer to seawater.This plays a role of indirect carbon fixation. Shellfish aquaculture fishery has obvious function of carbon sequestration.