Abstract: The goal of this study is to establish mathematical model for computing performance and analyzing the thermodynamic parameters variation of turbine/ramjet combined cycle engine during mode transition.Firstly,the computed flow charts of performance and thermodynamic parameters of combined engine,and the criteria of determining design parameters of combined engine were given.Secondly,according to computed performance and thermodynamic parameters along flight trajectory and design limitations,such as the Mach number at ramjet burner entrance＜0.3,the reasonable mode transition periods were optimized,and geometry parameters of critical section were given.Thirdly,the effects of opening area of mode transition valve on thermodynamic parameters,especially the relations of total and static pressure,were analyzed.The quantitative opening area of mode transition valve along flight trajectory were given for satisfying the criteria of thrust smoothing.At last,the thrust and specific impulse of turbine/ramjet combined cycle engine were calculated along flight trajectory.