Abstract: The high-coulomb two-electrode coaxial spark gap switch should be one of the better switching solutions in the power conditioning system for high power lasers. The turn-on process of this spark gap switch is studied with software ATP and PSCAD. The influences of several parameters on the turn-on characteristics are simulated and analysed, and then an electrical parameter model of the turn-on process is proposed. Based on this model, the relation between trigger voltage and turn-on energy is obtained in the resulting curve, which shows that trigger energy does not significantly affect the turn on of this switch.