Based on the closed-cavity mid-infrared laser, the temperature of the contaminant was measured when it was irradiated by a cw high energy laser with power intensity 3.16 kW/cm2. It was found that the contaminant achieves thermal equilibrium in a second and then the temperature stays at 1720 K. A physical model was established to describe the process of the thermal equilibrium. The mechanism of the mirror’s thermal damage was analyzed. It shows that the contaminant size plays an important role in the thermal damage of the optical mirror. Only when the contaminant size is smaller than a critical size (~20 μm), the contaminant may reach thermal equilibrium and the optical mirror work well in the high energy laser system. If the contaminant size is quite large (>200 μm), the optical mirror will be damaged under the irradiation of high energy laser. The results are of great help for improving the anti-damage capability of the mirror and maintaining the security of the high energy laser system.