Based on a microscopic molecular model, an analytical expression for the satura-tion intensity is obtained. By taking into account of the changes in gas pressure, temperature and flow velocity, the saturation intensity and small-sgnalgain coefficient at all points in the active region have been calculated under a variety of parameters. We have analysed and discussed the effects of the changes of pressure, flow velocity, gas composition and the discharge specific power on saturation intensity and small-signal gain coefficient. Finally, the calculated values for saturation intensity are compared with experimental results and good agreements are found.