The mechanism of Ni-catalyzed carboxylation of aryl C(sp2)-S bonds with CO2 was studied for the first time by density functional theory calculations. We first proposed another possible reaction pathway in which CO2 insertion occurs prior to reduction. Then, we performed calculations on all proposed reaction pathways, and our calculation results show that the pathway in which reduction occurs prior to CO2 insertion is the favored pathway for this reaction. Additionally, our calculations disclose that additive Zn0 acts in multifaceted roles. (1) Zn0 acts as a reductant to reduce the NiI and NiII intermediates. (2) The simultaneously formed ZnIIBr2 can undergo transmetalation with NiI or NiII intermediates to produce an aryl reservoir by forming arylzinc species. (3) ZnIIBr2 can also coordinate to the CO2 to lower the energy barrier of the CO2 insertion step. Moreover, the calculation results suggest that CO2 insertion is the rate-determining step of the reaction, and CO2 is easier to insert into the NiI-Ph bond rather than into the NiII-Ph bond. These calculation results can improve our understanding of the mechanism of the carboxylation process and the multifaceted roles of metal additive Zn0 and provide theoretical guidance for improving the carboxylation reaction.