The crystallization of KH2-xDxPO4 (DKDP) crystals with different deuterium contents ranged from 0 to 99% were investigated in solution by Attenuated Total Reflectance Infrared (ATR-IR) spectroscopy. The (H2PO4-)1-x(D2PO4-)x ionic groups were characterized by the υ1(PO4) and υ3(PO4) vibrations. The intensity changes of the υ1(PO4) vibration peaks and the broadened width changes of the υ3(PO4) vibration peaks indicate that the (H2PO4-)1-x(D2PO4-)x concentration increases with the measurement time increases. Besides, the appearance of δ(P-O…H/D-O-P) vibration illustrates the formation of (H2PO4-)n-x(D2PO4-)x clusters， which can be confirmed as growth unite.The intensity changes of the H-O-H and D-O-D vibration at the wavenumbers from 1 448 cm-1 to 1 653 cm-1can illustrate the un-uniform distribution of deuterium contents in DKDP crystallization process.