Abstract: The beam quality of unstable resonator of TEA CO2 coaxial output laser is deteriorated remarkably by the thermal accumulation of the output coupler. In order to improve the beam quality, a vortex tube cooling system of output coupler was adopted. The divergence angle was measured at an average pulse energy of 18.3 J under condition of different frequency with and without using the vortex tube cooling system at real time, and the timesdiffractionlimited factor was studied. The results show that the times diffractionlimited factor was changed from 3.52 to 6.94 without the vortex tube cooling system and from 350 to 3.88 with the cooling system at 3 Hz during an operating time of 300 s.