In the application of high-power laser long distance transmission, the far-field divergence is the one of important parameters to evaluate the laser performance. Conventional methods cannot be used to measure the far field divergence precisely because of the unique characteristics of TEA CO₂ laser, such as high average power, high peak power and large beam size. In order to solve this problem, a fitting analysis method was proposed to estimate the far field divergence of high power TEA CO₂ laser. Firstly, the far-field divergence of multimode Gaussian laser with large Fresnel number was calculated theoretically. Then, the near-field beam size(≤20 m) was measured experimentally using a spot ablated method, and the beam quality and waist radius were calculated based on the M² factor, and furthermore the far-field divergence was deduced. Finally, the results of these two methods were compared, and the deviation between the results was also discussed. It is indicated that the fitting method based on the experimental data is available to accurately evaluate the far-field beam divergence of high power TEA CO₂ laser.