Abstract: Two sets of propulsion concept were analyzed based the existent low bypass ratio augmented turbofan engine with mixed exhaust nozzle,in order to meet the mission requirements on the low Mach propulsion system(Ma≤3.5) in the hypersonic transportation.The result shows: as for the turbofan(TF) concept,by means of optimizing the control law of the base turbofan engine,total pressure difference between the two streams at the mixed zone was effectively reduced,contributing to flow organization at the afterburner inlet and flame stability.Also,the optimized control law contributes to lower specific fuel consumption.As for the turbofan/ramjet combined cycle engine concept with over-under configuration compared with the TF concept,independent inlet and nozzle were both required for the two engine models due to the aerodynamic mismatch during mode transition,which would increase the structure complexity,weight and technology risk.