Abstract: A two-dimensional scramjet forebody/inlet of mixed external and internal(compression) with plane lip and wedged lip were designed by using constant shock intensity method.The effect of temperature and shock/boundary-layer interaction was considered in the design.The design and off-design performance and flow field of several types of designed forebody/inlet were calculated and compared.The comparison results indicate at lower flight speed,the starting(performance) and total pressure recovery coefficient of forebody/inlet with wedge lip is better than that with plane lip.At high flight speed,the forebody/inlet with plane lip is of high pressure ratio and lower cowl drag,the forebody/inlet with wedge lip is of high total pressure recovery coefficient and high cowl drag.Due to the problem of unsteady interaction between combustor and inlet,the effect of back pressure raise in combustor on inlet/isolator flow field and start performance was(simulated).The research results present at lower flight Mach number,it is easy for combustion-(induced) pressure waves to propagate upstream and even to result in inlet unstart.The pressure(disturbance) results from chemical energy release in the combustor.Isolator can prevent the(pressure) disturbance from influencing inlet in a certain extent and is helpful to combustion mode transition.The ability for preventing pressure disturbance to propagate upstream improves when flight Mach number is increased.For an unstarted inlet,the captured mass flow will decrease quickly.The decrease extent to the captured airflow at high flight Mach number is larger than that at low flight Mach number.