The SnO2-ZnO-Ta2O5 based varistor ceramics was prepared by partial replacement of ZnO with MnO2. The effects of Zn and Mn co-doping on the microstructure and electrical properties of SnO2 were investigated. The experimental results showed that the substitution of ZnO with a small amount of MnO2 could improve the nonlinear performance of the samples and significantly increase the voltage gradient. When the MnO2 doping amount was 0.25%, the nonlinearity of the sample reaches 21.37, the voltage gradient was 422 V/mm, and the leakage current was 72.12 μA/cm2. The main reason for the change was that Mn supplements the original insoluble portion of ZnO in the SnO2 lattice, and undergoes the solid solution reaction to generate the acceptor defect ion Mn″Sn, which promotes the formation of the potential barrier. At the same time, Mn has low solubility in SnO2 lattice, so it was easy to segregate and precipitate on grain boundary layer, which hinders grain growth and increases voltage gradient.