An electric model of droplet charging process is constructed to explain theoretically the relatedness of instantaneous and unsteady charging voltage with the corona current in the droplet charging process during the electrostatic spraying. The voltage is depressing while the current is increasing rapidly, resulting in the phenomenon of inverse-Z type and the mechanism of drastic characteristics. The essence for this unstable phenomenon and the drastic characteristic lies in the transition of induction charging process to the corona charging process. The effect of droplet charging is affected directly by the structure of the pole, the charging mode and the dielectric properties of the liquid. The findings theoretically and experimentally reveal that there exists a linear relation between the droplet charging amount and the charging voltage in the induction-charging and the corona-charging processes.