This work studied the effect of oxygen vacancies in the Bi2O22+ and CO32- layers on the crystal structures, electronic and optical properties of Bi2O2CO3 with density functional theory (DFT) methods. The cell parameters and Bi-O bond lengths of Bi2O2CO3 before and after the introduction of oxygen vacancies were similar, so the effect of oxygen vacancies on crystal structures could be ignored. However, oxygen vacancies as electron doner provided electrons for the surrounding atoms, resulting in the change of charge distributions after the introduction of oxygen vacancies. Moreover, oxygen vacancies could reduce the band gap of Bi2O2CO3 and enhance the adsorption of visible light. This effect of oxygen vacancies became more obvious with the increasing of oxygen vacancy concentration. It was notable that the effects of oxygen vacancies in the Bi2O22+ and CO32- layers were different. Once a oxygen vacancy was introduced in the CO32- layer, a defect level appeared within the band gap of Bi2O2CO3. The oxygen vacancies in the Bi2O22+ layer were more easily formed in Bi2O2CO3 than that in the CO32- layer. Further more, the functions of oxygen vacancies in the Bi2O22+ layer for the reduction of band gap and the enhancement of visible light adsorption were more effective than that the oxygen vacancies in the CO32- layer, which was more obvious with the increasing of oxygen vacancy concentration. When the concentration of oxygen vacancies in the Bi2O22+ layer reached to 6.25%, the photoelectric properties of Bi2O2CO3 were the best. The DFT calculation results are consistent with the previous experimental results, and could provide insights into the mechanism for promoting the photoelectric properties of Bi2O2CO3 and other bismuth-based materials.