In order to analyze the thermal damage process under high energy laser irradiation, this paper focused on the high temperature condition and designed experimental device for coatings heating at different temperature. Ellipsometer was applied to measure the ellipsometric parameter (Ψ, ▵) of SiO<sub>2</sub>, HfO<sub>2</sub> and Al<sub>2</sub>O<sub>3</sub> single layer (prepared by electron beam (EB) evaporation on BK7 glass) at different temperatures, so the variation of optical constants with temperatures was obtained by inversion calculation. With the increase of temperature, the refractive index (n) of SiO<sub>2</sub>, HfO<sub>2</sub> and Al<sub>2</sub>O<sub>3</sub> single layer first decreased and then increased, the thickness (d) of these layers first increase and then decreased. However, the inflection point occurred at different temperature, and the amount of change was different for the layer with different material. The physical processes of thermal expansion and water evaporation were applied to explain the temperature dependent properties of the dielectric coatings.