The far-field light intensity model of annular plane beam through Kolmogorov atmospheric turbulence is presented, and both the discrete sampling of CCD pixel and detection noise are considered as well. The parameters of peak Strehl ratio、encircled energy Strehl ratio and beam quality factor are taken to evaluate far-field spot quality, and the Monte Carlo method is used to analyze the influence of four kinds of threshold method on calculation results of far-field spot quality when signal noise ratio (SNR) or turbulence strength is different. The results show that accurately measurement results of far-field spot quality are premised on adequately high SNR; if the SNR and turbulence strength are fixed, the threshold method of subtracting average background voltage of CCD and reserving random noise is the better choice to obtain the higher calculation accuracy of spot quality.