Accurate measurement of beam quality is very important in different areas of laser applications. Take the far-field Gaussian spot as an example, the calculation error formulas of peak Strehl ratio and beam quality factor beta are derived in the present of various error sources, such as background photon noise, CCD pixel's non-ideal sampling, CCD dark voltage, read-out noise of CCD, signal photon noise and measurement window size. The calculation errors of laser beam quality changing with various error sources are presented in theory and the corresponding numerically simulated results are given. Theoretical and simulated results show that when the threshold is set at (N-B) over bar + sigma(B), with (N-B) over bar standing for average value of the error sources and sigma(B) standing for mean-square value of the fluctuation in error sources, the lowest calculating error of laser beam quality could be achieved. Crown Copyright (c) 2012 Published by Elsevier Ltd. All rights reserved.