This paper focuses on the effect of aluminum additive on electrical properties of ZnO varistors. From the normalized current density versus electric field (J-E) curves, the influence of Al ionic additive on the leakage current and the residual voltage ratio of the sample is very intuitive. The barrier height of the grain boundary decreases with an increase in the Al dopant, but the donor density increases first and then decreases, which reaches the maximum while Al ionic 0.25% is added. The residual voltage ratio changes with the unevenness of ZnO grain sizes. Furthermore, the influence factors of the residual voltage ratio are analyzed.