The chemical kinetics of Cl/HN3/I2 laser has been simulated and the effect of the density of Cl, HN3 and I2 on small-signal gain has been obtained and discussed. The results show that the coefficient of smallsignal gain could be 1.6×10-4,11×10-3 and 1.1×10-2cm-1 at 400K when the initial density of Cl was 1×1015, 1×1016 and 1×1017cm-3 respectively, the initial density of HN3 and I2 for an optimal coefficient of smallsignal gain should be 1~2 times and 2%~4% of the initial density of Cl respectively.