Abstract: An unsteady numerical simulation was carried out on the start,unstart and restart performances of a fixed-geometry,axisymmetric hypersonic inlet due to changing free stream Mach number. The influence of cut-back side walls of the cowl on the characteristics of start and restart was analyzed. The results show that,the contradiction between captured mass flow and the flow capacity of the inlet is mitigated by lateral spillage in front of the cut-back side walls of the cowl,which improves the performance of start and restart. The strong interaction between shock wave and boundary layer in the internal duct is like a bottleneck to the start of the inlet.