Abstract: The specific hypersonic inlet models have been tested at Ma=4.0 wind tunnel to enrich the understanding of the effect of expansion waves on the inlet shoulder and the different compression ways on the inlet restart characteristics. The cowl angle and the thickness of the boundary layer have been regarded as the key influence factors. Results show that the expansion waves originated from the shoulder accelerate the local flow velocity and decrease the static pressure, which promotes the separation to move downstream. Thus, the inlet restart capability can be enhanced. And the multiple noncoalesced cowl shock waves can also improve the two-dimensional inlet restart capability. Due to the obvious three-dimensional structure of the separation induced by the swept shock, the inlet restart performance of sidewall-compression inlet differs from that of the cowl compression inlet. For the un-restart inlet caused by aerodynamic throat choke, the sidewall-compression can enhance the inlet restart capability effectively compared to the cowl-compression.