The raster scanning experiments for KDP crystal are carried out in R-on-1 mode at energy density below zero probability damage threshold to investigate the damage resistance capability as a function of pulse number after ultraviolet (UV) laser irradiation by using a tripled Nd:YAG laser. The1-on-1 damage measurements after laser irradiation indicate that the damage resistance of KDP is obviously enhanced and the increment is dependent on scanning number. The optical absorption and photoluminescence spectra show that the evolution of oxygen-vacancy centers in KDP crystal is important to the damage resistance capability. The Raman and infrared spectra show that the polarization distortions of PO4, P–OH and P＝O groups induced by UV laser irradiation also contribute to the changes of damage resistance.