The evaluation methods for far field optical quality of a TEA CO2 laser with a unstable resonator were researched. Based on the real data of the unstable resonator modified by the stable resonator of a high power TEA CO2 laser, three common theoretical evaluation methods were conducted to evaluate the optical quality.Obtained results were compared with that from the far field optical intensity distribution of a designed 2 kW unstable resonator laser. The analysis and comparison show that the unstable resonator can obtain a near diffraction limit and a high optical quality beam and the β factor is 4 times that of the stable resonator. Under the same power level, the far field power density of the unstable resonator is 19 times that of the stable one. Furthermore, the smaller block factor can obtain a higher power in bucket for the unstable resonator. It is concluded that the comprehensive prediction and evaluation of designed unstable resonator need to synthetically use the three theoretical methods to complete.