A one-dimensional numerical model for analyzing helium discharging of electro-optic switch driven by single-pulse process is proposed based on its microscopic physical mechanism. The influences of operating pressure on the characteristics of electro-optic switch driven by single-pulse process, such as gaseous breakdown, discharging current, charging voltage on KDP crystal, switching efficiency, switching speed, electron temperature, plasma density are numerically analyzed. Calculated results demonstrate that switching speed is obviously influenced by operating pressure, and operating pressure influences switching efficiency weakly. Under the same driven pulse, switching speed becomes quicker when operating pressure is enhanced in the case of low operating pressure, and it will be slower when the pressure is enhanced in the case of high operating pressure. It can be proved from results that optimum operating pressure is between 1000 Pa and 4000 Pa for electro-optic switch with an aperture of 8 cm×8 cm.