Program and method for simulating the three-dimensional flowfields of HYLTE nozzle and cavity, which governed by the compressible full Navier-Stokes equations with Realizable k-e turbulent model, has been upbuilt by using an coupled implicit FVM method based on efficient grid for complicated gecmetry. The HYLTE nozzle and cavity have been simulated as a integrative for considering non-uniform flowfield parameters of secondary nozzle exit. Cold flowfield parameters and mixing mechanism of reactant streams have been studied detailedly. Results show that mixing process is accelerated by two ways: reacting surface strentching and streamwise vortices. Furthermore,the cold result can be as the initial value of the reacting flowfields. The program can be modified to simulated overtone HF chemical lases and other kind of chemical lasers for studying the SSG coefficient and output power & chemical efficiency of FP reasonator.