Abstract: The parameters of the material of Al such as the thermal coupling coefficient conductivity, yield strength and so on, will change with the change of temperature induced by laser heating. It will influence the physical and mechanical properties of the material. The large thermal stress can result in the damage of the structure of material. Specially, in the case of the action of very large preload being present, the material structure can be yielded, deformed or fractured due to the action of the thermal stress and the preload, that is , the material structure may be damaged before the melt through of it. The damage effects of thermal stress in the thin plate of Al loaded induced by laser beam is calculated by using the method of finite element. The damage thresholds dependence on the preloads, laser power densities and wavelength are given out. The change of parameters mentioned above with the temperature is considered in the calculation. The results are in agreement with the experimental result given by Medford.