This paper firstly constructs the microstructure of the ZnO grain boundary layer of a high voltage system fits: microstructure characteristics of the ZnO varistor and conductive mechanism of the ZnO varistor. The ZnO structure can be seen as an irregular three-dimensional mesh composed of ZnO grain-high resistance grain boundary layer-ZnO grain. Then the electric field distribution of the ZnO grain boundary layer is calculated by Poisson’s equation, and the relationship curve of the space charge distribution inside the ZnO grain boundary with time is further simulated. Finally, the ZnO grain boundary performance is tested based on the bipolar carrier transport model with a double Schottky barrier, and the results show that the negative charge density at the grain boundary interface gradually decreases from 5×1025m−3 to 4.2452×1025m−3 and the initial charge value in the grain boundary region is 0m−3, and the negative charge at the grain boundary interface gradually decreases, and the negative charge in the grain boundary region gradually increases. In this study, the internal space charge distribution, Schottky barrier height and grain boundary leakage current are systematically investigated, which provides important theoretical support for the in-depth research on the conductivity mechanism, aging mechanism and performance improvement related to ZnO varistors.