The effect of Er2O3 doping on the microstructure and electrical properties of ZnO-Bi2O3-Sb2O3-Co2O3-MnO2-Cr2O3-SiO2 varistor was investigated. A part of Er is dissolved in the Bi-rich phase as doping Er2O3, which has a great influence on the grain boundary characteristics and electrical properties of ZnO varistors. The grain boundary resistivity decreases, the leakage current density increases, the double Schottky grain boundary barrier height and the nonlinear coefficient firstly increase and then decrease, and the breakdown field strength increases with increasing Er2O3 content from 0.09% to 0.35% (in mass fraction). The ZnO varistors obtained have a nonlinear coefficient of 54.4±1.5, a breakdown field strength of (470.1±2.8) V·mm-1, a leakage current density of (1.9±0.1) μA·cm-2, and a loss tangent of less than 0.03 at Er2O3 doping content of 0.27%, showing the optimum comprehensive electrical properties. The standard deviation of the performance parameters is small, indicating that the samples have a good reproducibility. This work provides a reference for the preparation of Er2O3-doped ZnO varistor ceramics with the excellent performance.