The damage resistance of 355nm laser can be improved for fluoride mirrors with an SiO<sub>2</sub> overcoat layer. Three kinds of samples are deposited with the overcoat layer of different thicknesses. Calculated with Stoney equation, the residual stress of the film can be altered to the compressive stress with the increase of the overcoat layer. Through the investigation of the damage pits, we find the damages are the thermal-mechanical coupled and induced by the nodules buried in the fluoride multilayers. The surface morphologies around the damage pits are analyzed from the two respects: Cracks and surface ablation. The two characters of the damage morphologies can be suppressed with the addition of the SiO<sub>2</sub> overcoat layer.