Although KDP (Potassium dihydrogen phosphate) crystal is very soft and brittle compared with diamond, the wear of diamond tool still exists in flying cutting, though the wear is much slower than that of diamond cutting copper and aluminum. The high technical requirements of KDP flying cutting has a small tolerance to tool wear. And the tool wear causes the quality of KDP processing uncontrollable, which results in low efficiency of processing and even workpiece scrap. In this paper, through the stress analysis of KDP lattice damage based on the first principle theory and the mechanical analysis of the cutting edge of diamond tool, it is proposed that the stress induces the wear of diamond tool in KDP flying cutting. The understanding of the wear mechanism of diamond tools lays a foundation for further research on the wear law of tools and for exploring the methods of prolonging the service life of tools.