The reaction mechanisms of the title reaction have been studied by using density functional theory (DFT) and Møller–Plesset perturbation theory (MP2). It is found that the reaction proceeds via two four-center transition states (TS1 and TS2) with a cyclic complex (b) locating between them on the reaction potential surface. The activation barriers of the two transition states are both negatively valued by -8.6 and -2.6 kcal/mol, respectively, at B3LYP/6-311+G level. Another reaction channel: ScS+ +CO2→ScOS+ +CO was also investigated and its activation barrier is much higher (50.2 kcal/mol) which is in line with the experimental observation.