A theoretical model is proposed to analyse the transverse-discharge flow type CO2 lasers. Theoretical expressions are obtained for predicting gain-saturation and output power of flow type gas lasers, these expressions are in agreement with the well-known formulae of non-flowing gas lasers, but the former contains correction terms related to pumping action in the upstream direction of the cavity and the gas flowing effect. Based on the analysis of the variation of intensity in the direction of flow, we derive the limiting values of stauration intensity and other parameters which were attained as the flow speed was being increased. Reliability of the semi-empirical analysis suggested, in literature 1-3 for the flow type laser performance predicted by choosing an average gain-coefficient anl adopting the formulae of non-flowing gas lasers has been proved. The qualitative analysis presented in article 4 is developed into a quantitative calculation. The present model should be used in place of the semi-empirical one, since the formulae deduced are as simple as those for the non-flowing gas lasers. The results computed for both amplifier and oscillator cavities using the present model are in agreement with experimental data 1, 18-21.