The safe operation of power equipment largely depends on the overvoltage protection level of the arrester. The ZnO varistors are widely used as the core components of the arresters in power systems because of the excellent nonlinear volt-ampere characteristics. In order to study the electrical properties of ZnO varistors under different external electric fields from the microstructure, the method of first-principles based on density functional theory (DFT) is used, and structure of ZnO/β-Bi₂O₃ interface containing zinc interstitial (Zn_i) and oxygen vacancy (V_o) defects is built. The results show that the V_o defect migrates after full relaxation. The Zn_i shifts to the interface under an external electric field. The interface energy increases rapidly after the electric field intensity has exceeded 0.1 V/Å, which means that the interaction force between the interfaces becomes larger, the distance between ZnO and β-Bi₂O₃ layers decreases, and the conductivity increases rapidly. The differential charge density, work function and Bader charge analysis method are used to calculate the barrier height at the interface, which proves that the built-in electric field is an important cause ingredient responsible for the non-linear volt-ampere characteristics of ZnO varistors. The effects of atomic orbital energy level, trap energy level and energy gap on the macroscopic conductivity of ZnO varistors are analyzed by using the method of density of states analysis. In this work are analyzed the different electrical parameters of the ZnO/β-Bi₂O₃ interface with aggregation defects by adjusting the intensity of the external electric field, and a new idea is provided for learning the electrical characteristics of ZnO varistors.