During the flashover experiments of parallel-plate electrodes in SF6, a double-peak phenomenon is discovered that a small peak occurs in the front-edge of the applied nanosecond pulse voltage which is originally smooth. In order to explore the cause of this phenomenon, the phenomenon is analyzed theoretically and ascribed to the discharge on the edge of the electrodes. The discharge enlarges the area of the parallel-plate electrodes, thus the equivalent capacitance of the electrodes is increased. The greater the change of the equivalent capacitance is, the more obvious the peak shows. Flashover experiments under different pressures are developed to validate the analysis. The results show that as the pressure increases, the peak is less and less obvious, and the amplitude of the small peak is higher and higher due to the suppression of the discharge in high pressure SF6, which is confirmed by the integrating images of the whole discharge process. Dendritic discharges occur on the edge of the electrodes during the flashover process. In low SF6 pressure, the stems are thick and bright, and generate many branches, but when the pressure is high, the number of stems and branches is reduced and the discharge channel also darkens.