The breakdown characteristics of trigger gap for the pre-ionization switch in N₂, the mixture of N₂ and SF₆ and SF₆ atmosphere under positive and negative pulse were investigated for the higher voltage grade and bipolarity application. The relationships between the breakdown voltage and delay time of the pre-ionization gap and the pressure of different kinds of gases were obtained, and the breakdown characteristics were compared for the pre-ionization gap under positive and negative polarities. The results of the experiment demonstrated that the breakdown process of the gap in N₂ was more stable, and the linearity of breakdown voltage varying with pressure in N₂ was the best among the three kinds of gases. The breakdown voltage was saturated only in SF₆ with the increase of gas pressure. The breakdown voltage of the gap under negative polarity was higher than that under the positive at some certain values of the pressure, and this phenomenon might suggest that there was a polarity effect upon the breakdown process of pre-ionization gap. Compared with the mixture and N₂, the absolute differences between negative and positive polarity for both breakdown voltage and delay time in SF₆ were relatively higher. N₂ should be preferred as the insulating medium for the pre-ionization gap to reduce the synchronization delay difference of the bipolar self-triggered switches. The polarity effect of the pre-ionization gap and the distinctions among the different kinds of insulated gas medium should be paid more attention in engineering applications for the pre-ionization gap.