The calculation method of conventional gasdynamic CO2 laser is always based on relaxation dynamics. In the paper, energy transfer processes in donor nozzle of mixing gasdynamic CO2 laser(MGDL) are treated as chemical reactions and freezing efficiency of N2(v=1) vibrational level is studied by using method of chemical reaction kinetics. Using forward reaction rate constants given by other papers, the backward reaction rate constants are calculated. Distribution of N2 (v=1) molecules in donor nozzle of MGDL and influences of total pressure and molar fraction of H2O on freezing efficiency of N2(v=1) vibrational level are studied. The results indicate that decrease of mass fraction of N2(v=1) molecules happens mainly in and around the nozzle throat. Compared with the case of not containing H2O, the freezing efficiency of N2 (v=1) vibrational level decreases remarkably while containing H2O. When total pressure increases, the freezing efficiency of N2(v=1) vibrational level decreases, but just a little. Comparatively, with increasing of molar fraction of H2O, the freezing efficiency of N2 (v=1) vibrational level decreases rapidly.