With the research on the next generation reusable launching vehicle, especially with the consideration of SSTO, more and more new technology ies under development. The tripropellant combustion is one of them. In this paper a thermal-dynamic model is developed and the theoretic performance of the tripropellant propulsion system is calculated and analyzed. The difference between performance of tripropellant and dual propellant system is discussed, and possibility of tripropellant system is valued.