The perturbation of middle and high spatial frequency caused by the polish process of the large aperture KDP crystal in high-power laser system has a certain impact on the performance of the facility. To study the effects of this kind of phase perturbation on the near-field uniformities and far-field focusing abilities, a calculation model which is based on fast Fourier transform (FFT) theory and optics wavefront data of SG-Ⅱ is established. The potential dangerousness which derives from the laser propagation to the facility is discussed.