In this paper, a single-gap pseudospark discharge chamber was built and tested with several electrode gap distances and pressures to explore the characteristics of the pseudospark under nanosecond pulsed voltages. Experimental results show that with the development of the discharge, anode plasma firstly forms near the anode, and then moves towards the cathode. The ionization initial voltage decreases with the increase of the electrode gap distance and pressure in our pseudospark discharge configuration. By analysing the emission spectra, it is concluded that the major emission spectra are composed of the second positive bands, and the electron energy of the plasma is in the level of several tens of electron volts. The mechanism of how the pressure and gap distance influence the characteristics has been discussed with the simulation of the collision ionization process using the Monte Carlo Collision method. Based on the simulation and experimental results, it is inferred that the breakdown voltages is decided by the ionization process of the electron with the neutral gas in the main gap, and a possible reason is put forward to explain the reason that U is a function of p(2)d.