Electromagnetic plasma accelerators which can generate hypervelocity and high density plasma jets have been widely used in the fields of nuclear physics and astrophysics. In this paper, an experimental platform of parallel-rail accelerator electromagnetically driven plasma is established, and the discharge modes under different discharge currents and gas injection conditions are studied through using magnetic probes, a spectrometer and an ICCD. A fast gas valve is used to inject argon into the rail electrode area. The time delay between the fast valve discharge and the parallel-rail accelerator discharge is fixed to be 450 μs. The waveform of power supply of the parallel-rail accelerator is a sinusoidal wave. The total capacitance is 120 μF, the total inductance is about 400 nH, and the maximum current is 170 kA. The fast valve current waveform is a double exponential waveform with a maximum current of 2.5 kA. When the discharge current is 40 kA, a current sheet with a certain thickness is generated, and the current sheet moves through different detection positions along the rail electrode at a certain velocity. Therefore, the working mode of the parallel-rail accelerator is the snowplow mode. As the discharge current increases, the trailing edge of the current channel is fixed during the current rising phase, and starts to move to the end of the rail during the current falling phase. A diffuse distributed current channel is formed, and the parallel-rail accelerator operates in a deflagration mode. As the gas injection mass increases, the current channel front velocity decreases to form a more concentrated distributed current channel, and the discharge mode turns into the snowplow mode. The stationary current channel in the deflagration mode is maintained mainly by ablating the electrode. The operating parameters mainly affect the rail voltage, which in turn affects the discharge mode of the parallel-rail accelerator. The rail voltage increases when the discharge current or the current rate of change increases. If the rail gap behind the current channel cannot withstand the high rail voltage under large discharge current or large current rate of change, the breakdown occurs, which results in the deflagration mode discharge.