Comparing with traditional stable resonator high-power TEA CO2 laser, it is always the key point to improve the optical beam quality as the same time guarantee the power of laser. Based on the original stable resonator, three sets of telescopic positive-branch confocal resonators is designed using unstable resonator scheme, which give the whole design process and theoretical analysis. The corresponding comparison experimental study are conducted with single-pulse energy, divergence angle and pulse duration time. Experimental results indicate that using unstable resonator can largely improve the divergence angle as the same time to maintain the high single-pulse energy. The single-pulse energy 13.7J, the divergence angle 1mrad (stable resonator as 2.4mrad) and pulse duration time 50ns (stable resonator as 98ns) are obtained from the best unstable resonator. This work will make this kind of laser to be widely used in the field of laser manufacture.