Abstract: The experiments of unstable resonator were performed for compressing divergence angle through modifying a high-power stable resonator TEA CO2 laser. The virtual confocal positive branch was adopted to study the influences of structure parameters and mode volume on output energy and optical beam divergence for the unstable resonator. Twenty sets of mirrors with four magnifications and five mode volumes were designed and manufactured. In the same experimental conditions, the maximum pulsed energy of the unstable resonator is 14 J, which is 70% of that of the plano-concave stable resonator, but the divergence angle is just a quarter of the latter’s.