An unstarting flow in an axisymmetric model scramjet isolator was investigated at a Mach 4.5 flow by using planar laser Rayleigh scattering flow visualization and wall pressure measurements. Unstart was triggered by jet injection that mimicked combustion-driven downstream flow choking. Flow features such as unstart shockwaves, thickened boundary layers, lambda shock, and quasi-steady and transient motions were observed during unstart processes. Two wedged inlets of 6 and 9 degree were used to study the effects of an incident shock on unstart. The threshold of unstart for the 9-degree wedged inlet was lower than that for the 6-degree wedged inlet. The results from the pressure measurements showed that the propagation speed of the unstart shockwave firstly became faster as the jet momentum flux increased. However, further increase in the jet momentum flux slowed down the propagation of unstart shock. The quasi-steady and transient motions of the unstart shockwaves were observed in pressure measurements. The quasi-steady motions occurred adjacent to the impingement location of the incident shockwave on the boundary layer. It was discussed that the presence of a pressure gradient induced by an incident shockwave is attributed to the quasi-steady and transient motions.