The ZnO-Pr6O11-Co2O3-Cr2O3-Er2O3-SiO2 varistors were prepared by a solid-state sintering method. The effect of SiO2 doping on the phase, microstructure, varistor property, and impedance characteristic of the ZnO varistors was investigated. The results show that the grain size decreases gradually with the increase of SiO2 content due to the restraining effect of grain growth in SiO2. The ZnO varistor with SiO2 content of 1.0% (in mole fraction) has an optimum overall performance to provide overvoltage protection for circuit (i.e., the most second phase generated, the maximum breakdown voltage of 435.5 V/mm, the maximum mean grain boundary voltage of 1.63 V, the maximum nonlinear coefficient of 17.5, and the maximum resistivity of grain boundary of 17 400 MΩ·cm, the maximum barrier height of grain boundary of 0.37 eV, and the minimum leak current of 1 μA, respectively). The breakdown voltage and nonlinear coefficient of ZnO varistor with SiO2 content of 1.0% are 3.6 and 6.6 times greater than those of ZnO varistor without SiO2.