Photocatalytic CO₂ reduction is one of the important means to alleviate the energy crisis. In this work, an oxygen-linked and brown graphitic carbon nitride (GACN) was successfully prepared by thermal polymerization after oil bath method. GACN introduced oxygen atoms on surface of BulkCN. Various characterizations of the material show that the prepared GACN has a different structure and higher photoelectronic activity compared to BulkCN. GACN possessed strong photocatalytic CO₂ reduction capacity, and the photocatalytic activity was significantly improved compared with BulkCN. In view of density functional theory calculations, it is proved that the oxygen atoms introduced by GACN increase CO₂ photoreaction reactivity, enhance electronic activity and reduce the reaction energy barrier. This work can have a positive effect on the photocatalytic application of g-C₃N₄ with the existence of oxygen atoms.