The dynamic process of the thermal distortion of the silicon mirror has been studied theoretically and experimentally. The displacement of the suface of the silicon mirror is varying rapidly in the beginning of the distortion. When the power of the incident laser is 200 W and the absorptivity of the silicon mirror is 70%, after two seconds the deflection of the mirror introduced by thermal stresses reachs 0.76 μm already. It is not a megligible problem for the high power short wavelength laser.