Abstract: A big progress has been made for scramjets in the past 60 years. However, whether it has enough thrust or not is not clear to date. In this paper, the thrust performance of the nozzle was analysed by using the isentropic expansion theory, because the thrust is mostly produced by the nozzle in scramjets. The parameters in the combustor were used as the boundary conditions. A dimensionless thrust equation was obtained by integrating the pressure along the nozzle wall. The key parameters and physical laws governing the thrust performance were discussed. The analysis results show that the supersonic combustion is good for producing higher thrust. The main way to get heavy thrust is to increase the combustion static pressure in the combustor. The analysis results are in good agreement with CFD results, which demonstrates its correctness.