The large aperture deuterated potassium dihydrogen phosphate (DKDP) is an important frequency conversion crystal in a large power laser device. There are many defects inside the DKDP bulk material, including the varying element impurities and electronic defects. Comparing with the defect-free material, these bulk defects can easily absorb incident laser energy and pose the risks of initiating damage sites when exposed to high-energy lasers. Beside bulk defects, there are surface defects originating from the DKDP machining process, including cracks, scratches and protuberances. These surface defects affect the damage performance of DKDP crystal by increasing light absorption and weakening local mechanical strength. Due to the defects from both bulk and surface, the actual damage threshold of DKDP crystal is much lower than the expected theoretical value. The lack of its laser damage resistance seriously restricts the laser output power. In this study, the off-line sub-nanosecond laser conditioning technology is proposed to effectively improve the laser damage performance of large aperture DKDP crystal. Its principle is to irradiate DKDP with a mild laser fluence in advance. Although the laser pretreatment cannot directly eliminate the impurities, dislocations, grain boundaries or other macro structural defects in crystals, it indeed changes the distribution and density of intrinsic point defects on a micro-scale. It suggests that the complicated reactions of electron-hole, atom-vacancy and the intrinsic point defect annihilation caused by the microstructural transformation of crystal materials under laser conditioning are the possible reasons for reducing absorption and improving the damage resistance. In this experiment, the result of the damage to high-power laser device shows that the mean surface damage density of DKDP crystal at 9 J/cm ²decreases from 5.02 pp/cm ²to 0.55 pp/cm ²after sub-nanosecond laser conditioning. The laser conditioning can remove the protuberance defects on the surface, thus reducing the surface damage density. In addition, the damage size probably decreases after laser conditioning. There is a leftward shift in the damage size curve after laser conditioning, and its peak decreases from 25 μm to 18 μm–20 μm. In addition, due to the removal effect of laser conditioning on defects, the spatial distribution of damage points after sub-nanosecond laser irradiation turns more uniform. This study provides a foundation for the applications of off-line sub-nanosecond laser conditioning technology in high-power laser facility.