Abstract: An engine flow path of a base geometry consists of the elaborately designed isolator and a detonation combustor with a length to diameter ratio of 20, along with a comparison group of four different geometries, were studied numerically by means of single detonation to investigate the propagation characteristics of back-pressure in an air-breathing pulse detonation engine. Parameters of the back-pressure, including propagation speed, pressure peak, and its decay rate as well as the total pressure recovery coefficient of the base model were considered and discussed. The results demonstrate that the designed isolator is able to reduce the back-propagation speed and the back-pressure peak effectively. The detonation combustor with a larger length to diameter ratio contains more fuel and oxidant, which needs more efforts to prevent the back-pressure. The decay rate of the pressure peak is mainly affected by the geometry of the isolator at the early stage of the back-propagation process, and afterwards it depends on the distance of the back-propagation. When the inlet pressure is given, the detonation combustor with a smaller length to diameter ratio has a more rapid exhaust process, and therefore, a slighter back-pressure propagation phenomenon. Large total pressure losses are not found in the designed isolator when the Mach number of the incoming flow is between 0.15~0.80 under sea level conditions.