A simplified model for calculating concentration field of diffusion-type CW HP chemical lasers is presented. The velocity field is assumed to be analogical to one of the ordinary flat-plate laminar boundarys. The temperature field and concentration field are solved by coupling the energy and species conservation equations as well as the velocity similar solution. A contour of the mixing boundary is obtained. In addition, the effects of velocity, temperature of the gas flow and chemical reactions on HF concentration of the excited states are studied.