In this paper, the general case of gas laser cavities is shown when the diffusion of molecules is so fast that the spatial holes burned are almost washed out, and the light intensity superposition can be used in the stimulated emission terms and stimulated absorption terms. Thus the physical foundation of the light intensity superposition is laid by virtue of the concrete analysis of a 003~N3~H20 system, it is pointed out that except some particular cases, the light intensity superposition can always be used to calculate the field distributions and power outputs for general CW gas laser.