For a high power CO 2 laser, its work medium and resonator components including mirrors and coupler will be distorted under the condition of high power laser output. Although with various cooling methods for the resonator, the distortions always happen under different output powers. It means the Rayleigh range Z R of the certain resonator is varying under different output power. The change rule of Rayleigh range Z R of output beams caused by the output coupler thermal deformation at various output power are researched. An adaptive optics system, which can compensate the change of Rayleigh range Z R and revises the effect caused by the thermal distortion of the output coupler, was used. Good focusing characteristics in flying optics is achieved in this way.