Excimer, Nd:YAG, and CO2 laser are used to pre-irradiate KDP crystal samples at subthreshold energies. The change of damage threshold produced by laser treatment is investigated. Excimer laser treatment can effectively raise the bulk damage threshold of KDP crystals, YAG laser pre-irradiation can raise it a little, but cw CO2 laser treatment makes it descend. The improvement of laser treatment differs with the laser wavelength, the laser power, and the total deposited energy. Laser treatment can also improve the surface characteristics of KDP crystals, but the effect is not as good as that in the bulk. It is deduced that laser treatment can restore the stress field remains, the micro-cracks, and the dislocations derived from crystal growth and polish processing. However, it has little effect on the defects caused by impurities. These produce the different effects under various treatment conditions and the difference between interior and surface in improving the characteristics of materials by laser treatment.