Abstract: Used optical interferometry,the dynamic process of thermal distortion of the silicon mirror was investigated experimentally,while the 2kW CO2 laser beam is used as incident source.The diplacement of surface of the silicon mirror is varying raplidly in the beginning of the distortion.For a silicon mirror,that is 70mm in diameter and 8mm in thickness,the maximum deflection introduced by thermal distortion is 0.76μm,when the absorbed laser power is 140W and the acting time is 4s.It is not an negligible problem to a high power short wavelength laser.