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[Display omitted] The Potassium dihydrogen phosphate (KH 2 PO 4 : KDP) of ferroelectric phase is a polarized structure that can be used for nonlinear optical transformation. Using the generalized gradient approximate (GGA), we study and predict the crystal structural trends, electronic structures, charge densities, and optical properties of KDP crystals with paraelectric (PE) and ferroelectric (FE) phases under pressures ranging from 0 to 20 GPa. We have studied how pressure affects the structural and electronic properties of KDP. It is found that under high pressure, KDP undergoes a transition from Fdd 2 structure to I -42 d structure. The calculations indicate that when the hydrostatic structural pressure of the ferroelectric KDP is applied to 11.36 GPa, the KDP has a paraelectric phase structure. The 11.36 GPa is the critical pressure value at which the structure undergoes phase transition. We further showed that as the pressure increases, the lattice size and ion displacement undergo a strong distortion. Surprisingly, the band gap of KDP greatly increases with the increase of pressure. The results show that high pressure enhances the compactness of the KDP system and enhance the laser damage threshold, which can open a new way for high pressure manipulation of frequency doubling materials in optoelectronic applications. [ABSTRACT FROM AUTHOR]