On the basis of the theory of indentation fracture mechanics, the relationship between the surface defect area and median crack of potassium dihydrogen phosphate crystals were analyzed theoretically. In the machine tool parameters and spindle speed in certain circumstances, the potassium dihydrogen phosphate crystal was processed by single point diamond turning under different cutting depth and feedrate. And then the ratio coefficient of unit area defect on the surface of the crystal was calculated. The experimental results show that the depth of surface defect is proportional to the ratio coefficient of area, which is consistent with the theoretical analysis, so the estimation method for the depth of surface defects by calculating the ratio coefficient of the surface defect area was processed. Finally, the high efficiency cutting process based on this method was proposed, the obtained arithmetic mean height value of surface roughness is better than that of 5 nm by the actual processing.