KDP crystals takes on anisotropy which will effect cutting force and machining impact,when cutting in different angles.The KDP crystals cutting forces testing equipment was constructed by three direction force measuring instrument,and the cutting forces were measured along different crystals direction.It is indicated that the cutting forces are distributed disciplinarian as given angle,the numerical value of the force is anastomosed with theory result.At the same time,surface roughness is presented typical periodic distribution on different crystals direction.Based on this measure test,experiment on optimizing surface roughness was developed.Ultra smooth surface machining with Sq＜1.8 nm on workpiece 50 mm×50 mm is achieved.