In a Mach 1.8 duct flow, a pseudo-shock wave was produced by a plasma jet torch using a N2 and H2/N2 mixture as a feedstock and the schlieren photograph, the static pressure on the wall of the test section and stagnation pressure at the exit of the test section were measured to investigate the influence of combustion on the pseudo-shock wave and its formation process. On the other hand, the case injected the large mass flow rate was simulated using three-dimensional Navier-Stokes equations. In Mach 1.8, when the nitrogen is used as the feedstock of plasma jet torch, three types of the flow field were observed and the pseudo-shock wave observed in the present research was l-type. The pseudo-shock wave produced at the upstream of the plasma jet torch depends on the condition near the torch but not downstream of the test section. The combustion promotes the formation of the pseudo-shock wave but does not influence on its formation mechanism. The formation mechanism is assumed by the top wall pressure distributions and numerical results.