In view of the voltage dip/swell in the power system, a programmable power supply was used to modulate the applied voltage, and the effect of voltage fluctuation on the internal partial discharge characteristics of epoxy resin was analyzed. The results show that the voltage fluctuation will lead to the change of partial discharge pattern of air gap, which exhibits asymmetry in positive and negative half cycles. The greater the applied voltage fluctuation, the more significant the asymmetry of partial discharge. The reason for the change of partial discharge pattern is due to the combined effect of external electric fields and surface charges accumulated inside the air gap. On the one hand, the electric field generated by surface charges can overlap the applied electric field to adjust the total electric field in air gap. On the other hand, as the discharge continues, the dissipation and neutralization of surface charges can also affect the total electric field in air gap, resulting in the partial discharge exhibit asymmetry in positive and negative half-cycle of the applied voltage.