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Theoretical studies on CdS/BiOI heterojunctions are lacking; therefore, this paper studies their electronic structure and optical properties through density functional theory calculations. First-principles calculations were made to investigate the effect of introducing oxygen vacancy defects on the photocatalytic efficiency of CdS/BiOI. The band gap of CdS/BiOI was effectively narrowed compared to that of its constituent materials, resulting in a redshift of the absorption edge. Based on the obtained work functions, the difference in Fermi energy levels caused a spontaneous flow of electrons, resulting in band bending and the establishment of a local electric field. The electric field promoted the separation of electron–hole pairs. At the same time, the addition of oxygen vacancies created impurity levels, allowing light absorption to extend to the infrared region, further promoting carrier separation. Our study reveals the origin of the enhanced efficiency of CdS/BiOI heterostructures and also predicts the photocatalytic activity of CdS/BiOI heterojunctions after introducing oxygen vacancies. These results may facilitate the design of such structures with enhanced photocatalytic properties. • The electronic structure and optical properties of CdS/BiOI heterojunction are studied by using density functional theory, and the effect of introducing oxygen vacancy defects into the structure is predicted theoretically. • According to the band structure, density of state, 3D/2D charge density difference and work function calculated by the first principle, the electron transfer from BiOI to CdS can be analyzed, and the trend is enhanced after the introduction of oxygen vacancy defect. • The calculation of optical properties shows that the formation of heterojunction and the introduction of oxygen vacancy defects promote the enhancement of optical absorption intensity. • This paper supplements the theoretical calculation of vacancy in CdS/BiOI heterojunction. And defects are introduced into the heterostructure to enhance the catalytic performance of such materials. [ABSTRACT FROM AUTHOR]