Abstract: Based on the analysis of the far-field intensity distribution,the beam quality of unstable resonators has been studied in detail.Ithas been proven that the intensity in the far-field of unstable resonato rs is the zeroth order Bessel beam distribution,if the obscure ratio ε→1.N umerical calculation and analysis have shown that the "power in the bucket" curve is a suitable method used to characterize the beam quality in the far-field of unstable resonators.Meanwhile,the shortcomings of this method have been poin ted out.