The damage of optical elements is a main limiting factor for a large laser system. The understanding of damage mechanisms can promote system design, choice of operational parameters and technology development. Thermodynamics damage of optical coatings induced by absorbing inclusions was studied based on thermal irradiation model. It is shown that the initial damage time of optical coatings during a laser pulse is very short and the most part of the laser pulse generates larger damage. Considering the phase change of absorbing inclusion, we calculated the gas pressure exerting on coatings due to the gas phase change of absorbing inclusion and discussed the possibility of the macro damage of optical coatings. This model can well interpret the flat bottom pit damage morphology of optical coatings.