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KDP crystal is showing a good property in high-power laser systems. However, working in a high-power environment is easy to have damaged-defect. Dehydration of KDP crystal is one of the damage phenomena. We explore the total energy and physical properties of the KDP crystal progressive dehydration by using First-principles calculations. It is found that the band gap of the KDP crystal gradually decreases with the deepening of dehydration, and there are many obvious defect states between 4 eV and 8 eV (the corresponding wavelength region is from 310 nm to 155 nm). It indicates that dehydration causes a reduction in the damage threshold of the KDP crystal. Our results indicate that these defect states are due to the change of hybridization type of P atoms, which is gradually transformed from original sp3 hybridization to sp2 hybridization in the dehydration process. An obvious redshift can be observed in the absorption spectrum, producing many distinct absorption peaks. All of the results can provide the good basis for deeply understanding the electronic and optical properties of the KDP crystal.