The scale inhibition mechanism of carboxymethyl dextran (CMD), a new green scale inhibitor, was studied via quantum mechanical calculations and molecular dynamics (MD) simulations. Specifically, the interactions between the functional groups of CMD and calcite (104), (110), and (1–10) surfaces in aqueous solution were modelled at different degrees of polymerization (DP) of CMD (2, 4, 6, 8, and 10, respectively). The adsorption configuration, radial distribution function, deformation energy, binding energy, and relative concentration distribution of water molecules near the surface of calcite were calculated. The results showed that the carboxyl functional groups in CMD were strongly electronegative and able to form strong chemisorption bonds with calcium ions on the calcite surface. This can change the regular arrangement of the surface and prevent the combination of carbonate particles and calcium ions to form calcite scale. Furthermore, as the DP increased, the binding energy and the peak value of the radial distribution function increased, indicating an increase in the probability and strength of adsorption of CMD on the calcite surface, which can further enhance the scale inhibition efficiency. Display omitted •Quantum mechanics combined with molecular dynamics simulation was used to study the scale inhibition mechanism of CMD.•The carboxyl functional group was the most electronegative functional group in CMD.•Enhanced chemical bonding occurred between the carboxyl functional groups and calcium ions.•The scale inhibition efficiency increased with the degrees of polymerization of CMD increased.•The scale inhibition efficiency of CMD on surfaces were in the order of calcite (104) < calcite (110) < calcite (1−10).