High average power high repetition rate TEA CO2 lasers have many important applications such as laser manufacturing, laser ranging and military applications, especially its optical quality is the key point for its applications. Based on original high power TEA CO2 with stable resonator in this paper, three sets of telescopic positive-branch confocal resonators are designed and manufactured using unstable resonator scheme. The corresponding comparison experiments are performed with single-pulse energy and divergence angle. Experimental results indicate that using unstable resonator can largely improve the divergence angle and compress the pulse width as the same time to maintain the high single-pulse energy. The single-pulse energy 13.4 J, the divergence angle 1.2 mrad and pulse duration time 49.5 ns are obtained from the best unstable resonator. At the same time the output mirror freezing by change of state can guarantee the average power of laser greater than several kW at high repetition rate.