The intrinsic damage mechanisms for optical thin film in vacuum environments is investigated through the inter- coupling process between optical film and environment and the thermal- mechanical damage process between laser and optical film. It is found that water vapor desorption of the film layers exposed to vacuum environment results in the decrease of the refractive index and the thermal conductivity of the film layer and hence leads to the serious decrease of laser- induced damage threshold for e- beam evaporated porous coatings in vacuum.