The output mode of a TEA CO 2 laser is a “flat top” type rather than Gauss distribution predicted by the conventional theory. In this paper, the formation mechanism is investigated. The energy distributions of the output beams in different degree of saturation are measured while the other conditions remain unchanged. It has been demonstrated experimently that the intense saturation due to the high gain in the TEA CO 2 laser is respossible for the output mode′s widening and uniformizing.