Abstract: The secondary mass flowrate of the HYLTE nozzle in DF/HF chemical lasers was numerically and theoretically calculated, the mass flowrate coefficients of D2，H2 and He with 0.01~0.4MPa stagnation pressure and 0.1~0.9mm height of the half throat are obtained. The changerules of mass flowrate coefficient with stagnation pressure and half throat's height were got, the functions deduced from these rules were of satisfactory preciseion. As a result, some suggestions are presented on the design and maintenance of secondary nozzle.