The lattice strain of large-scale KDP crystal is characterized by using high resolution X-ray diffraction technique, and the lattice stress is analysed quantitatively in detail. The results indicate that KDP crystal may cleave easily along the [001] direction, which corresponds to the cracking phenomenon in the practical work. The major factors of introducing the internal stress and causing crack in the crystal growth process are summarized. Those conclusions provide important theoretical basis for proposing crack control measures of the large-scale KDP crystals.