Abstract: In this paper, a plane flying-cutting machine tool for KDP crystal is developed. The linear axis of the machine is driven by linear motor and supported by hydrostatic guideway. High stiffness air-bearing spindle and high stiffness spindle rotating mechanism is adopted for tool spindle. Based on the high resolution position feedback and linear driver and PID control algorithm, the low-speed fluctuation of 0.018 mm/min and the position accuracy of 0.01 μm at 1 mm/min speed are obtained. The tool micro-feeding device is realized by differential thread feeding, and the high precision feeding with a feed resolution of 1 micron and a position displacement of less than 1 micron after locking is finally obtained. After optimizing the process parameters, the experiment of diamond flying-cutting machine tool on 100 mm×100 mm ×10 mm KDP crystal shows that the surface roughness Rq is better than 2 nm. High precision index of surface PV value better than 3 micron is obtained after processing 400 mm × 400 mm × 30 mm aluminum mirror.