Abstract: This paper analyzed the nonlinear relationship between the peak power of the output and the gain coefficient of the B-class CO2 laser with experimental and theoretic methods. The results show that when the modulating velocity nearly equals to the rate of transition between the rotation levels , the peak power of the output laser will be saturated at a gain coefficient point. This is the very specialize character of the B-class CO2 laser, the result of our experiment agreed with that of simulations well.