A 100 J TEA CO2 laser was employed to study the effects of variety of pulse energy from 13 J to 80 J on the momentum coefficient for air-breathing mode. The results indicated that the momentum coupling coefficient (Cm) had no obvious change when the pulse energy decreased from 80 J to 24 J. While the energy was under 22 J, the Cm declined 52%. A preliminary analysis of theory was given, and an experiment of changing the air pressure in the vacuum chamber was carried to verify that.