Based on beam propagation theory, the quantitative relation between PSD (power spectral density) of "defects" distribution on optics and that of near field intensity was studied. Its validity was discussed on linear medium, nonlinear medium and spatial filter. The results show that PSD of "defects" distribution on optics and PSD of near field intensity are connected by a system propagation factor when the modulation is weak, which is determined by system architecture and operation state. These results provide a theoretical basis for specifications of "defects" distribution on optics, and also a guidance to enhancing the power of high power solid-state laser system.