The photo carboxylation of the benzylic C(sp3)-H bond catalyzed by xanthone/nickel were examined by density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations. This study corroborates the previous proposal that light promotes the H-transfer from benzylic C(sp3)-H bond of the p-methoxytoluene to excited state of photocatalyst xanthone. Meanwhile, Ni(0) catalyst could mediate the H-transfer to occur via an electroncoupled-proton transfer manner, and then remarkably facilitates the carboxylation step (compared to the Niabsent systems). After that, the generated Ni(I) intermediate and ketyl radical anion complete the carboxylation and electron transfer processes independently.