Laser induced damage in optical components is a limiting factor to the improvement of high-power laser. Using K9 glass irradiated with the CO2 continuous-wave laser as an example, the thermal and mechanical damage mechanism of optical material induced by continuous-wave laser is investigated. The equations of heat conduction and thermoelastic dynamics are solved through integral-transform method. Transient distribution of temperature field and thermal stress field is described. It is found that the definition of the thermal diffusion length is not applicable when incident laser is Gaussian beam; therefore, the dependence of the site of the maximal thermal stress on irradiation time is derived by curve fitting method, then the analytical solution of the damage threshold intensity is calculated. Because the stress damage threshold intensity is smaller than the melting damage threshold intensity, permanent damage in optical material is produced when circumference stress exceeds the tensile strength. The damage morphology of K9 glass induced by laser is proved to be cleavage burst. The model exhibits good agreement with the experiment result.