Abstract: According to the work mechanism of the pre-cooled TBCC (Turbine Based Combined Cycle), the pre-cooled TBCC function mathematics model that considering calculation of the alterable specific heat was set up, and the homologous calculation software was programmed. Taking the certain small scaled turbojet as an example, the influence by mass injection and pre-cooled engine was calculated and analyzed along fly trajectory. The calculation results show that, similar to the SteamJet given by the United States, adopting water injection and pre-cooled method can expand the regulations turbojet work scope consumedly, and can satisfy the request of the supersonic aircraft or two-stage-to-orbit aircraft of the first stage equip.