In this paper, the effect of impurities in the vacuum chamber during the deposition of optical coatings on laser induced damage threshold (LIDT) of films is analysed. The quantity of impurity is tested by second ion mass spectroscopy (SIMS). By comparing the testing result, it shows that getting rid of every kinds of factors which produce impurities in the thin films is a promising way to improve the LIDT of optical coatings.