The planar thermal conduction of Al-film and three-dimentional thermal conduction in K9 glass (substrate) when an Al-film reflector was irradiated by laser beam were analysed. The distribution of temperature and thermal stress in the K9 glass was calculated using finite element method. The influence of reflectivity and transmission distance of laser on the temperature increment and thermal stress of the reflector was also discussed. The transmission distance should be chosen appropriately to keep the reflector out of crack.