To study the influence of impurity size on the damage threshold of anti-reflection film, by using photolithography technology, cylindrical Al impurities with different diameters and thickness of 100nm were added between the fused quartz and the anti-reflection film. Comsol was used to understand the damage process, and the 532nm nanosecond laser was used for threshold test, then the experimental data and the sample damage morphology are analyzed, the results show that the damage thresholds of the films with impurity diameters of 50um, 100um, 200um, 300um and 400um are respectively 18.93J/cm2,18.62J/cm2,17.11J/cm2,15.28J/cm2,13.47J/cm2,which goes down nonlinearly, and the main cause of film damage is that the "impurity absorption point" absorbs laser energy to generate heat, and the axial thermal stress generated by conduction to the anti-reflection film exceeds the tensile strength of the film.