Abstract: In experimental facility of cylindrical gain generator assembly for HF chemical laser, primary diluent He is injected into combustor from two different regions. Some of He is injected by D2 jet orifices after being premixed with D2. The other of He is injected by backward He jet orifices at back end of primary nozzle blade. The effect of changing mass flow rate of onward He or backward He on laser’s characteristics is studied by a couple of lasing experiments. The experimental results show that, as compared with changing mass flow rate of onward He, changing mass flow rate of backward He can acquire much higher output power and better ignition stability. When primary diluent ratio is required to be increased, changing mass flow rate of backward He should be given priority.