Abstract: The switch used in FLTD primary energy-storing system is chosen as the study object. A self-breakdown experiment of the switches with different gap length has been done at the same breakdown voltage by changing the gap pressure. The influence of the gap length on the switch performance has been researched by observing the change of the breakdown voltage distribution. The experiment result shows that, the self-breakdown voltage dispersion at 6 mm gap is the least. Breakdown voltage distribution with the gap length equal to or less than 6 mm is similar to Gauss distribution function and it is similar to extreme distribution function with the gap length larger than 6 mm. The switch with a long gap whose breakdown voltage distribution loses the symmetry has better static performance at low working ratio. According to the electrode structure used in this paper, the gap length should be chosen between 6 mm and 9 mm to get the best static performance. The reason for the change of breakdown voltage distribution is the alteration of the electric field influencing the electron emission on cathode.