The coaxial gun discharge could produce plasma jet with high velocity, high density and high energy density, which has a wide range of applications in plasma space propulsion, simulation of the interaction between edge local mode and wall materials in ITER, fuel injection in magnetic confinement fusion devices, and laboratory astrophysics, etc. In the pre-filled discharge mode, the plasma current sheet is formed near the insulating layer surface and moves toward the end of the coaxial gun under Lorentz force. Plasma velocity, density and purity characteristics are very important research contents for the actual application of coaxial gun. Emission spectrometry as a non-interference method can be used to diagnose a variety of plasma physical properties. In this experiment, the effects of different discharge currents and gas pressure on the plasma dynamics, electron density and impurity emission spectra of coaxial gun discharge plasma are studied through the measurement of plasma photocurrent, emission spectra and the shooting of discharge images. The experimental results show that the acceleration time of the plasma in the gun decreases with the increase of the current in 30~70 kA when the gas pressure is 10 Pa, the spectra of anode and cathode impurities in plasma increase with the current amplitude. When the discharge current is 40 kA and the gas pressure is in 10~70 Pa, the acceleration time of plasma increases with the increase of gas pressure, and the spectral intensity of the cathode impurity in the plasma decreases with the increase of the pressure, while the spectral intensity of the anode impurity increases gradually, but its growth rate decreases continuously. According to the analysis, the presence of metallic impurities originating from the electrode material limits the jet velocity of the plasma and is the main cause of the deviation from theoretical value. The plasma pinch effect at the nozzle of coaxial gun and the acceleration time of high-density arc in the gun are important factors affecting anode ablation. The impurity of cathode material is produced by ion bombardment sputtering, which mainly depends on the energy attached to ions. Therefore, a reasonable choice for discharge parameters is the key factor to obtain optimal plasma characteristics during the discharge of the coaxial gun.