Firstly, primary structure and operation process of a hybrid propulsion system of small-scale turbojet engine and pulse detonation engine (PDE) were introduced.Secondly, the simplified mathematical model was established to calculate its performance.Finally, fight performance and effect of efficiency characteristics of turbojet engine were analyzed.Results indicate that the optimal compressor ratio is 5 ~8, and the optimal turbine inlet temperature is 1 200~1300 K to reduce the specific fuel consumption and to increase the specific thrust.Presetting the inlet pressure of PDE as const, thrust decreases and total specific fuel consumption increases with Mach increasing and flight altitude decreasing; Specific thrust of PDE shows little change but total specific fuel consumption shows significant rise when efficiency of compressor and turbine decreases.