Abstract: Heat energy loss fraction and corresponding equilibrium temperature of combustor are roughly estimated according to the heat transfer theory on condition that input fuels mass flow rates, compositions proportioning and relevant configuration parameters of combustor are known. Entrance boundary parameters of primary nozzle are then determined. The flow field in coupling region and optical cavity of annular hypersonic low temperature (HYLTE) nozzle is numerically simulated using 3D computational fluid dynamics method. Spatial distributions of F atom mass fraction, static temperature and pressure are described. Small signal gain coefficient distributions for spectral lines in optical cavity are calculated by post processing of the flow field data. The experiments of measuring output spectrum and moving optical axis demonstrate the rationality of the computational model considering heat energy loss of combustor walls.