In the pressure recovery system of a high-energy chemical laser, the ejector is a very important equipment. Its efficiency influences all the system. In this paper, theory of the engineering calculation of the ejector efficiency is studied. The conservation equations are used. Since in an ejector, active and passive air has diffierent thermodynamic and physical constants. This influence must be considered in calculation. The results of the calculation of diffierent parameters show that some parameters have important influence. Some qualitative rules have got for engineering design.