The formation reasons and suppression methods of the typical defects generated on the surface of the potassium dihydrogen phosphate (KDP) crystal are investigated. The correctness of the analysis results of the formation reasons are confirmed via the fly-cutting fabrication and the surface painting fly-cutting experiments, and the formation process of the surface defects of the KDP crystal are further clarified. Theoretical models which are suitable for the description of the formation process of the surface defects of the KDP crystal are established and the process conditions for the realization of the defect-free KDP surfaces are shown. The fly-cutting fabrication parameters and the cutting tool structures are optimized, and thus the validity of the suppression methods is verified. The research results show that, the variance range of the brittle ductile transition (BDT) depth of the (001) plane of KDP crystal is 125-268 nm under the fly-cutting conditions. When the cutting is along 45° direction, the BDT depth reaches the maximum. At this point, the fracture pits can be avoided on the crystal surface as long as the feed rate does not exceed 36.6 μm·r -1. Besides, the convex defects are eliminated by the optimization of the cutting tool structures, the surface defects of the KDP crystal can also be suppressed effectively, and thus a smooth KDP surface with a roughness less than 2 nm is obtained.