Benzoin is an important feedstock with high additional value for its extensive use in chemical industry. Benzoin condensation characterized by C-C coupling between benzaldehyde requires the use of nucleophilic catalysts including cyanide or N-heterocyclic carbene and is restricted to organic medium such as MeCN, diethyl ether, etc. Construction of efficient and non-toxic catalysts for benzoin synthesis still remains a challenge. Herein, highly selective (97%) benzoin synthesis from benzyl alcohol was achieved over potassium modified g-C3N4 via light-driven tandem selective oxidation and C-C coupling. The outstanding performance was attributed to alkali modifications on the electronic structure and surface chemical environment of g-C3N4.K+ intercalation not only facilitated the light harvesting as well as the transport of charge carriers, but also induced surface deprotonation of g-C3N4 and thus remarkable nucleophilicity for prompting the C-C coupling reactions. This work sheds light on the design of earth-abundant inorganic photocatalysts for C-C coupling reactions such as the green synthesis of benzoin under ambient conditions.