Herein, the breakdown mechanism of alumina‐based ceramics (Al2O3–ZrO2) is studied. The specimens are characterized for their density, phase structure, micromorphology, breakdown process, and breakdown channel by micro computed tomography, high‐speed cameras, etc. The study reveals that the dielectric breakdown stability of the samples shows a decreasing trend and the dielectric loss shows an upward trend with the increase of ZrO2 content. The tetragonal phase zirconia is beneficial to enhance the dielectric breakdown strength of the sample, which is related to the martensitic transformation to prevent crack propagation. Before the ceramic breakdown, the insulating oil first breaks down, causing electric sparks. Once the ceramic breakdown occurs, the charges are rearranged, and a large amount of heat energy is generated instantly to melt the substance around the breakdown channel and erupt to the surface to form pits under the action of high internal stress.