During the coating process, the metal contamination in the vacuum chamber is one of the significant factors to reduce the optical properties of thin films. The gas composition during coating process was tested by high-vacuum residual gas analyser. It is found that when applying brass pedestal in the vacuum chamber, zinc will sublime from the pedestal under high temperature and deposit in the thin film, thus reducing its laser-induced damage threshold (LIDT). The composition of the coatings was studied by surface analysis technology which proved the zinc impurity in the coatings. And the laser-induced damage experiment also showed that its LIDT was distinctly reduced.