CdS/CdMnTe heterojunction is the core of photoelectric conversion of CdMnTe film solar cells, whose interface properties have an important influence on the cell efficiency. In this study, the first-principles calculation method based on density functional theory is used to build the surface model for each of the CdS (002) and the CdMnTe (111) and the model of CdS/CdMnTe heterojunction with Mn atoms occupying different positions, and to analyze their electronic properties and optical properties. The results show that the lattice mismatch of the CdS/CdMnTe heterojunction is about 3.5%, the atomic positions and bond lengths of the interface change slightly after relaxation. The density of states shows that there is no interface state near the Fermi level in CdS/CdMnTe interface. Besides, the atoms at CdS/CdMnTe interface are hybridized, which can enhance the interface bonding. The differential charge density analyses indicate that the charge transfer mainly occurs at the interface, and electrons transfer from CdMnTe to CdS. The optical analysis shows that CdS/CdMnTe heterojunction mainly absorbs ultraviolet light, and the absorption coefficient can reach 10 ⁵cm ^–1. However, the optical properties of heterojunctions with different Mn atom positions are slightly different. In a range of 200–250 nm, the absorption coefficient of the heterojunction with Mn atom in the middle layer is larger, but in a range of 250–900 nm, the absorption peak of the heterojunction with Mn atom in the interface layer is higher. The results in this paper can provide some references for improving the photoelectric conversion efficiency of stacked solar cells through the reasonable construction of the heterojunction model and the analysis of the interface photoelectric performance, which is beneficial to the experimental research of multi-band gap heterojunction.