Abstract: Polytetrafluoroethylene (PTFE) and epoxy fiberglass board (a common PCB substrate material) are two commonly used insulating materials in spacecraft. Addressing the issue of insulation failure due to surface flashover in these materials under low pressure conditions, the influence of low gas pressure and electrode spacing on the flashover discharge threshold was investigated through experiments in this paper. The results show that when the gas pressure is below 0.1 Pa, the flashover voltages of PTFE and the epoxy fiberglass board are approximately 40 kV and 28 kV, respectively. Within the pressure range of 0.1 Pa to 1 Pa, the flashover voltages of both materials decrease rapidly as the pressure increases. When the gas pressure exceeds 1 Pa and rise until 100 Pa, the flashover voltages of both the materials decrease to approximately 600 V and 1000 V, respectively, thus significantly increasing the discharge risk. This study reveals the flashover behavior of insulating materials at low-pressures, providing insights for optimizing the design and application of spacecraft and space device insulation materials to enhancing their performance against flashover risk.