The modeling and performance calculation for the deeply precooled combined cycle engine—SABRE(synergistic air-breathing rocket engine) were carried out. A steady componentlevel model of the engine was constructed. The rule of performance variation of the engine in the air-breathing mode along the flight trajectory was obtained, and the altitude-velocity characteristic of the engine was studied. The model is reliable with thrust error less than 6%, and it can be used to calculate performance of SABRE under the air-breathing mode. It is indicated that the SABRE is characterized by both large thrust as rocket engines and high specific impulse as aircraft engines, and its specific impulse is between 21 300~27 380 m/s; along with the increase of altitude and velocity, the thrust and specific impulse of SABRE increase first and then decrease. By decreasing the inlet airstream temperature with precooler, the range of height and velocity of SABRE can be enlarged to 25 km and 5Ma, which is required by hypersonic flight; the lower limitation of velocity is decided by the maximum mass flow rate of compressor, and the higher limitation is decided by the operation rule of helium loop.