Abstract: The DF/HF chemical laser needs the pressure recovery system with huge volume to work successfully. In order to optimize the design of the laser and the pressure recovery system, the flow field and total pressure losing of two types nozzles(standard nozzle and thin neck nozzle) in DF/HF chemical lasers was studied by numerical simulation. The difference of their total pressure losing and interior flow field were compared. Then the influence of the difference on the entire laser system performance was discussed. The computer result shows that the total pressure losing of the thin neck nozzle was as same as the standard nozzle. So, using the thin neck nozzle was benefit to diminishing the volume of gain generator of the DF/HF chemical laser. And it did not take more berden to the pressure recovery system. So, the pressure recovery system at the end of the laser will not have to be enlarged.