A turbofan engine comprises all structures of a turbofan engine such as a fan, an outer duct, an axial-flow air compressor, an evaporative combustion chamber, a turbine and a tail jet pipe. The turbojet engine is characterized in that a bypass pipe and an inner exhaust valve are further included, an outer exhaust valve is arranged or not arranged on the bypass pipe between an air inlet and an air outlet of the bypass pipe, after the engine is started and operates, the inner exhaust valve is opened, by opening and closing of the bypass pipe, the inner exhaust valve and the existing or nonexistent outer exhaust valve, the supercharge ratio of the axial-flow air compressor or the fan is increased and kept unchanged, the increased supercharge ratio is not higher than the limitation borne by the fan, the axial-flow air compressor and a turbine according to the mechanical strength, turbine inlet temperature is kept unchanged, namely the supercharge ratio and thrust-weight ratio are increased, oil consumption rate is lowered, the levels of the fan, the axial-flow air compressor and the turbine are not increased, and the turbine inlet temperature is not raised.