Gas film cooling,for protecting the gain generators,and two-step combustion method,for improving combustion efficiency,were applied together to continuous wave DF/HF chemical laser. Portion of the main diluent-helium was transferred to inject at the converging section of the primary nozzle. The flowfield in cavity was numerically simulated and the results showed that spectra′s small signal gain had a small amplitude increase. The results did not exhibit high combustion efficiency as that in traditional two-step combustion method,and the high temperature gas flow demanded the combustion chamber to reach a much higher level. The total temperature could be lowered through increasing oxidizing gas flow and the gain coefficient was increased.