The laser damage mechanism for thin films on zinc-germanium diphosphide (ZGP) crystal is studied experimentally and theoretically to enhance the laser induced damage threshold (LIDT). ZGP which grows at one time is cut into 6 pieces, and is grinded and polished simultaneously. Four of the samples are coated with antireflection multi-layer optical thin film and are analyzed by scanning electron microscope (SEM), it is found that the thin film is rather smooth while some clusters still exist on some areas of the thin film. Secondary ion mass spectrometer (SIMS) is also used to ascertain that the clusters contain platinum. Laser whose wavelength is 2 μm and pulse width is 31 ns is used to test the thin film on ZGP according to the 1-on-1 method of ISO11254 standard to measure the LIDT of the thin film. The results show that the average LIDT of those coated samples is 0.68 J/cm2. The LIDT is also calculated by finite element method and the result is 3.2 J/cm2. The study shows that platinum impurity has significant impact on the LIDT of optical thin film on ZGP and therefore decreasing the impurity density will increase LIDT of the thin film.