"Plasma driven micro-particle accelerator" is a ground device for simulating impact effects of small debris in space. The particle velocity is determined mainly by axial velocity of plasma in a coaxial gun. Emission spectrometry is used to study the plasma axial velocity at different voltages and gas pressures. The experimental results indicate that axial velocity increases with the increase of discharging voltage, and doesn't change significantly with the pressure of working gas, which is consistent with the result of numerical simulation. This result is useful to improve the plasma axial velocity further, and provides an experimental basis for optimizing the accelerator.