From the view of crystal chemistry, the growth units and forming mechanism of KDP crystals were investigated. The thickness of boundary layers changing with lattice faces was studied by in-situ observation with Raman spectra, where the solution structure during crystal growth could be well observed. It was found that the solution structure at the boundary layer is similar to that of the crystal, so that the growth units are proposed as [H2PO4]-. The difference of growth rates of lattice faces is determined by the combination of growth units, which is mainly dominated by the angular orientation of K-O and H-O bonds. Consequently the reason about the difference of growth rates of lattice faces and the influence of pH value on the crystal morphology can be well elucidated.