Wide bandgap semiconductor materials have received more and more attention because of their unique properties and potential applications. Single-doped tin dioxide (SnO2) has been studied extensively, however the calculation of SnO2 doped with Sb and S is less involved. Co-doping can effectively improve the solubility of the dopant, increase the activation rate by reducing the ionization energy of the acceptor level and the donor level, and increase the carrier mobility at low doping concentration. Co-doping can solve the problem that is difficult to solve with single doping. Based on the density functional theory of the first principle and the plane wave pseudopotential method, in this paper we study the electronic structure and electrical properties of SnO2 doped with Sb and S by using the generalized gradient approximation algorithm. The geometrical optimization calculation is carried out for the doped structure. The Broyden-Fletcher-Goldfarb-Shanno algorithm is used to find the stable structure with the lowest energy. The plane wave cutoff energy is set to be 360 eV, and the internal stress is less than or equal to 0.1 GPa. By analyzing the electronic structures, it is found that the material is still direct bandgap n-type semiconductor after being co-doped. The electron density is changed, and the overlap of atomic orbital is enhanced. It is conducive to the transfer of electrons. New energy levels are observed in the energy band of co-doped SnO2, and the bandgap width is narrower than that of single doping, thus making electronic transitions become easier. Fermi level is observed in the conduction-band, which leads to the metal-like properties of the material. The electronic density of states is further investigated. The results of the density of states confirm the correctness of electron transfer. In the middle of the valence-band, the hybridization is found to happen between the S atomic orbital and the Sn and Sb orbitals. The top of the valence-band is occupied by the S-3p orbit, thus providing more hole carriers to move up to the top of valence-band. With the increase of S doping concentration, the bandgap and the width of conduction-band both continue to decrease. As a result, the conductive performance turns better.