In the present paper, we have prepared a multi-layer grating on a layer of gold film. The gratings laser damage threshold and damage morphologies have been tested using an 800 nm wavelength, 450 ps laser pulse. The gratings laser damage threshold is 0.95 J/cm(2). Based on the tested damage characteristics and the calculated temperature field and stress field in gratings, the gratings damage mechanism has been analyzed. In theoretical calculation, the ionization process of dielectric layers in gratings and the distribution as well as evolution of the dielectric absorption coefficient have been considered. It can be concluded that the gratings damage process is thermal ablation action dominated.