Abstract: The flow filed in the isolator of a typical hypersonic inlet was simulated with uniform and non-uniform(with oblique shock) inflow.The pressure-temperature-Mach number at isolator exit relationship was investigated,the relationship of each other was presented,and all of those were independent of the isolator length.The predicting methods of the performance for constant area isolator under back pressure were studied,and doable predicting method of isolator under back pressure was presented.The concept of maximum pressure capability was developed when the back pressure was greater than the maximum pressure capability with unstarting of inlet.It's observed that the maximum pressure capability has relationship with the Mach number of the isolator exit with no back pressure.The predicting methods of maximum pressure capability for constant area isolator were presented.The maximum pressure-isolator length relationship was also presented.