Abstract: The distribution of neutral propellant gas pressure in the heaterless cathode，especially the region which along the cathode tip to keeper，is important to the ignition voltage value.In this paper，the pressure and velocity distribution before the ignition of heaterless cathode were simulated by the FEA method and high Mach number compressible fluid model.The result is indicated that the neutral gas pressure along the cathode tip and keeper was related sensitively to the keeper orifice diameter and it was effect weakly by the cathode tip orifice diameter and the spacing along the tip and keeper.But the ignition voltage of heaterless cathode will be affected by the spacing along the electrode according to the Paschen's law.The result of this paper demonstrates that key factors of influences the breakdown voltage of heaterless cathode are keeper orifice diameter and spacing along the tip and keeper when the propellant is not changed.That would be given some