Abstract: Due to the significant influence of metal impurity ions on the damage properties of crystals and the limitation of experimental conditions, the effect mechanism of Fe and its cluster defects remains unclear. In this paper, Fe and its cluster defects in KDP and ADP crystals are simulated by the method of first-principles, to determine their effects on crystal structure and optical properties. It is found that Fe atom entered into KDP and ADP crystals mainly by replacing P atom to form FeO₄ group, and the relatively stable form is Fe³⁺. In addition, the magnetic condition has little effect on the structure and energy of the crystal, and the damage threshold of the crystal is mainly affected by the obvious optical absorption in the range of 200−300 nm. The cluster defects form when there is impurity of Fe, which could be recombined with V_O through charge compensation, but hardly with V_OH. The influence of the cluster defects is mainly the effect of Fe on the crystal structure and properties.