The rapid densification mechanism for flash sintering (FS) is still debatable. In order to clarify the densification process, in this study, 3 mol% yttria-stabilized zirconia (3YSZ) was selected as a model material to systematically investigate its sintering stress and microstructural evolution under the applied current during the current-surge stage of FS. The results indicate that the driving force for rapid densification in current-surge stage of FS is electrostatic interaction resulting from the accumulation of charged defects on the free surfaces of adjacent particles under an electric field. The electrostatic attraction causes adjacent particles to rotate to the close orientation and aggregate to densification. The microstructural observations indicate that the grain of flash-sintered 3YSZ is composed of many sub-grains with sizes close to those of initial particles. Such characteristics of flash-sintered 3YSZ further confirm that the rapid densification process results from the electrostatic attraction. Based on these achievements, the densification kinetics equation was proposed as well.