To overcome the issue of UV-light response character of Bi2O2CO3 due to its wide band gap, we primarily attempted to understand the possibility of improving the photocatalytic activity of Bi2O2CO3 via g-C3N4 surface-decoration by the theoretical calculation. Subsequently, g-C3N4 surface-decorated Bi2O2CO3 was successfully prepared via a facile hydrothermal method. It was found that the g-C3N4 surface-decorated Bi2O2CO3 samples exhibited enhanced activities for the photodegradation of tetracycline compared with pure Bi2O2CO3 upon simulated solar light irradiation. Among them, the 10 wt% g-C3N4 surface-decorated Bi2O2CO3 sample showed the highest photocatalytic efficiency. First principle calculation and experimental data confirmed that the charge transfer at the interface between g-C3N4 and Bi2O2CO3 could significantly suppress the recombination of photogenerated electron-holes pairs, thus improving the photocatalytic performance. The proposed mechanism for the enhanced photocatalytic activity was also discussed. Moreover, the photodegradation of antibiotics over g-C3N4 surface-decorated Bi2O2CO3 was also performed in actual water matrix.