Abstract: Based on isentropic assumption, mass conservation and separation prediction formula, an engineering estimation method of nozzle performance considering flow separation was developed for large expansion ratio nozzle, and the performance evaluation method was verified with numerical simulation in this paper. The variation trend of thrust performance with nozzle pressure ratio(NPR) estimated by present method is in good agreement with simulation results. The results indicate that the present method has a good predicting ability at highly over-expanded flow conditions and an accurate estimation on large NPR state. It can provide direct guidance for nozzle design.