Based on the density functional theory, the electronic structure, charge density and defect formation energy of Mg-N anion-cation co-doped SnO2 were calculated by first-principles principle. Replaced Sn and O with Mg and N in SnO2, respectively, with doping concentrations of 4.17at% and 2.08at%, the covalency between Mg-N is significantly higher than that of Sn-O. Under oxygen-rich conditions, the formation energy of Mg-N co-doped SnO2 is 2.67 eV, which can be effectively substituted by the acceptor. The band gap increases when Mg doped SnO2, and the Fermi level enters the valence band, but the band gap is narrowed with Mg-N co-doped SnO2, and shows a p-type conductivity.