The safe operation of power equipment largely depends on the overvoltage protection level of the arrester. 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 (Zni) and oxygen vacancy (Vo) defects has been built. The results show that the Vo defect migrates after fully relaxation. Zni shift to the interface under the external electric field. The interface energy increases rapidly after the electric field intensity exceeds 0.1V/?, which means 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 methods are used to calculate the barrier height at the interface, which proved that the built-in electric field is an important reason 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. This work analyzes the different electrical parameters of the ZnO/β-Bi2O3 interface with aggregation defects by adjusting the intensity of the external electric field, and provides a new idea for learning the electrical characteristics of ZnO varistors.