Two dimensional flowfields in different nozzles, with equal section throat area and different section lengthhave been simulated. The change of sonic line with the section length, and optimum lengthof the section have been given. The flow fields and the performance of three nozzlesdesigned with different methods, have been simulated and analysed. Results of simulationshow that the new method can optimize/promote the performance of nozzle, such as lateralvelocity, the efficiency of “freezing” fluorine atom and longitudinal velocity, and also can preventboundary layer separation and shock wave. Moreover, thickness of boundary layer has beencalculated according to the velocity field obtained from simulation. Developing tendencyof boundary layer in the nozzle of DF/HF chemical laser has been discussed.