The functional crystal material Potassium dihydrogen phosphate KH2PO4 (KDP for short) is widely used in navigate, spaceflight, national defenses, energy sources and information technology fields because of its excellent non-linear optical property. However, KDP crystal has some disadvantages such as soft, fragile, hygroscopic, anisotropy, thermally sensitive and low rigidity, which make it a most difficult processing material. In this paper, KDP crystal was machined by the wheel grinding. Some equipment, such as Optic microscope, SEM, ZYGO and Raman spectroscope were used to observe and analysis the KDP crystal surface which was damaged during grinding. The results shows that serious nicks, crashes and the press stress appeared on the surface of the KDP crystal after grinding.