A special simulation model for the time-domain responses of ZnO varistors is proposed in this paper. This model is based on the Voronoi network and contains the actual conduction mechanism of the grain boundaries, as well as the capacitances of the grain boundaries. The AC responses of the actual varistor samples were simulated and compared with the experimental results. The simulated waveforms agree with the measured ones very well, which indicates that the simulation model is quite reasonable.