This study investigates the influence of GeO2 on the nonlinear coefficient (α) and breakdown voltage (EB) of TiO2–Ta2O5–CaCO3 varistor ceramics doped with GeO2 and prepared according to a conventional ball milling–molding–sintering process. The electrical performance parameters, including the nonlinear coefficient (α), breakdown electric field (EB), and leakage current (JL), were assessed using a varistor direct-current instrument. The average barrier height ΦB of each sample was calculated using the relevant formula. Analyses conducted by X-ray diffraction, scanning electron microscopy, and scanning transmission electron microscopy demonstrated that GeO2 doping changed the microstructures of TiO2–Ta2O5–CaCO3 ceramics, resulting increased α and decreased EB values. Ceramics doped with 0.2 mol % Ta2O5, 0.2 mol % CaCO3 and 0.9 mol % GeO2 exhibited the maximum α value (α = 9.8) and the highest grain boundary barrier (ΦB = 0.92 eV) but a low EB value (EB = 17.2 V·mm−1).