In order to study the thermal characteristic of output window during the working process of high power CO2 laser, a time-dependent model coupled with heat transfer and structure mechanic was established to calculate the distribution of temperature, thermal deformation and thermal stress. First, the physics characteristic of output window materials was discussed. Then, based on the resonant cavity structure of 10 kW TEA CO2 laser and the physics characteristic of output windows, a heat transfer and structure mechanic model was established. Finally, COMSOL finite element software was used to calculate the distribution of temperature, thermal deformation and thermal stress. The influence of different materials of output window on parameters was analyzed. The results indicate that the temperature rise of ZnSe output window is less than the GaAs output window when the output power is consistent for both materials; the deformation of output windows is in the range of μm dimension under the 10 kW laser power condition; the distribution of thermal deformation and thermal stress is more even in GaAs output window for its high heat transfer coefficient.