Numerical simulation and performance analysis of flow fields in hypersonic low temperature (HYLTE) nozzle and optical cavity of a small continuous wave (CW) DF chemical laser, which employed a single throat HYLTE nozzle (STL), under different stagnation and structural parameters have been carried into execution to interprete the disparity between device design and experiment. Distribution of F atom mass fraction, static pressure and static temperature in cold flow field has been analyzed. Distribution of small signal gain in reactive flow field also has been simulated and qualitatively compared with spots ablated from organic glass. Some orientations have been conformed to improve the performance of the STL DF laser.