Abstract: This paper presents the design of a new low-gas-pressure gas switch that uses an external particle source for generating the original ionized particles, using Geant4 program to build the physical model of the switch. Verification shows that the model is reliable and effective. The model is adopted to calculate the effect of gas species and gas pressure on electron gain, get the minimum injected-electron number for forming self-sustained discharge. The simulation results show that the electron gain of nitrogen is considerably larger than that of helium, which is in accordance with the higher ionization energy of helium. The electron gain increases with increasing gas pressure in the form of nonlinear growth. To form self-sustained discharge, the density of injected-electron is (1-2)×10⁵/cm².