Abstract: Optical system of Shenguang-Ⅲ works under high power condition and demands that optical elements are highly resistant to laser damage. Micro-defects in optical element may cause enhancement of electric field in small area which will lead to lower laser damage threshold. In this paper, Time-domain finite difference method(FDTD) is used to simulate distribution of electromagnetic field. Quantitative judgement of selection of computation parameters is given. The results show that if micro-defects(such as pore, impurity, etc) exist in the optical material, the intensity of electric field is enhanced by several times.