For the nozzle throat ablation problem existing in the gain generator of high power DF/HF chemical lasers, there is a way called film cooling, to protect the wall and the throat of the nozzle. The differences of flow characteristic are numerically simulated under the different mass fluxes of the helium film and the influence of film on main stream is analyzed. The relationship between the throat width of the helium film, the main stream and the nozzle′s throat width is important for the design of nozzle′s throat with gaseous film cooling through the slot.