Deciphering the dynamic evolution of double Schottky barriers (DSBs) during high-temperature sintering remains a fundamental challenge in establishing structure–function relationships in ZnO varistor ceramics. In-situ dielectric spectroscopy is employed to continuously monitor capacitance and conductance throughout the entire sintering process, with measurement fidelity ensured by rigorous decoupling of system impedance. During sintering, a pronounced transition to negative permittivity in ZnO varistor ceramics is directly observed above 700 °C, originating from electron plasma oscillations within the semiconducting bulk. The apparent activation energy (E_a) of direct current (DC) conductance is identified as a robust descriptor for tracking DSB evolution. The results reveal a temporal decoupling between microstructural densification and electrical functionalization: Grain growth is largely completed during the heating and soaking stages, whereas DSB formation predominantly occurs during the cooling stage. Specifically, from 800 to 650 °C, E_a increases from nearly 0 to approximately 1.4 eV, accompanied by an increase in grain boundary resistance (R_gb), indicating the progressive formation of DSBs. Scanning electron microscopy–energy-dispersive X-ray spectroscopy (SEM–EDS) line scans of quenched samples further reveal grain boundary enrichment of Bi and Sb, accompanied by the redistribution of Mn and Ni after soaking and during cooling, supporting the concurrent evolution of the grain boundary chemical environment. Upon further cooling, DSB formation suppresses the free-carrier response, R_gb increases markedly, and the negative permittivity gradually disappears. Meanwhile, E_a decreases slightly from 650 to 600 °C and exhibits a distinct peak in the 500–600 °C range. In-situ X-ray diffraction (XRD) shows that this feature coincides with the gradual disappearance of β-Bi₂O₃ and the increasing dominance of α-Bi₂O₃, suggesting that the Bi-rich phase transition contributes to the subsequent modification of DSBs. By directly linking in-situ electrical behavior to microstructural evolution during sintering, this work provides a deterministic framework for the rational design and performance optimization of functional electroceramics.