THz time domain spectrum of KH2PO4 crystal in the range from 0.2 to 1.6 THz, the far-infrared spectrum in the range from 50-4 000 cm-1, and ultraviolet-visible-infrared spectrum in the range from 200-2 000 nm were measured at the room temperature. The energy band gap Eg of KH2PO4 Crystal is 5.91 eV. There is a wide band of the phonon absorption in the KDP crystal. The values of the absorption coefficient are 35-80 cm-1 in the range from 0.2 to 205.5 THz. The end of the low frequency of the optical phonon model is smaller than 0.2 THz. The highest frequency νLO of the longitudinal optical phonon model LO is about 205.5 THz, and the force constant K of H—O ionic chemical bond stretch vibration from the ωLO was calculated to be 13.13 N·cm-1.