To figure out the causes of cavity mirror defects in a Chemical Oxygen-iodine Laser (COIL), the surface defects on the cavity mirror were studied. A Scanning Probe Microscopy (SPM) was used to observe the surface defects on the cavity mirror, and its micro topography was analyzed. The shapes of the surface defects and the reasons why the defects appeared were discussed. Then, a simplified model of cavity mirror with defects was established. The ring beam irradiating the cavity mirror was simulated by using COMSOL Multiphysics. The relationship between the defect size, power density and the surface temperature of the cavity mirror was given and the influence of an adsorption layer on the melting damage of the cavity mirror was analyzed. The calculated results show that when the defect area is a constant, the larger the laser radiation power is and the faster the temperature grows, the more the melting damage of film surface is. Moreover, the mirror film has the possibility of melting damage at the defect radius of 2.3 mm. The absorption coefficient of the absorption layer increases by 1%, then the maximum temperature of the mirror increases about 210 K. The conclusion provides a reference for the analysis of the causes of cavity mirror damage and the replacement of the cavity mirror in the COIL system.