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In this work, the effect of Bi2O3 content on the microstructure, mechanical, thermal, and electrical properties of ZnO varistors was investigated, and the high impulse current withstand properties and failure mechanism of ZnO varistors was discussed in detail. When content of Bi2O3 equaled to 0.75 mol%, the varistors possessed excellent mechanical, thermal, and electrical properties, where the value of σf and Ef were 130.77 MPa and 69.60 GPa, KT and λT were 6.504 W·(m·°C)−1 and 7.00 × 10–6 °C−1, E1mA, α, JL, and K were 272.13 V·mm–1, 55.96, 1.55 μA·cm–2, and 1.74, respectively, being due to the highest density and finest uniform grain size distribution. Finite Element Analysis (FEA) showed that the thermal stress decreased first and then increased with the increase of Bi2O3 content, where Ef and λT played a more important role than KT. And the loaded impulse current Ip can generate Joule heat to improve the temperature of ZnO varistors, thereby increasing the thermal stress and promoting the migration of dopants ions from ZnO to the Bi2O3 crystal lattice. Hence, JL increased and U1mA reduced, which further increased the thermal effect generated by Ip. When the value of Ip equaled to 100 kA, high temperature produced a loose microstructure and broken grains originated from the strong thermal stress. Meanwhile, the enhanced migration increased JL (3.77 μA·cm−2) and decreased U1mA (4.93 kV), resulting in a critical value of ΔU1 mA (9.88%). A further increase in Ip led to the failure of ZnO varistors. [ABSTRACT FROM AUTHOR]