Abstract: Net thrust and supersonic combustion oscillation are two crucial gas physical issues to scramjet engine, which have not been solved to date. In this paper, the classical CJ detonation theory was first used to analyze the performance of scramjet engine. Results show that the static temperature of air at the combustion chamber inlet influences the pressure of supersonic combustion very much. When the propagation velocity of the combustion wave is greater than the velocity of air in the isolator, it propagates upstream and causes unstart of scramjet engine. The flight Mach numbers of Ma=6~8 are in criti-cal unsteady combustion region for scramjet engine. For flight Mach number Ma>9, the combustion flow field of scramjet engine becomes steady.