Laser-induced damage often determines the effective lifetime of an optic in large high-energy laser facilities.We present the damage performance on the rear surface of a large-aperture KDP crystal for 351 nm,5 ns laser pulses.Surface damage shows a lower threshold than bulk damage after conditioning.Craters initiated on the scratch are found to increase with the shot number before filling the scratch.The experimental results reveal that damage initiation is mainly caused by extrinsic nanoabsorbers buried in the surface during the large-aperture laser operation. Laser-induced damage often determines the effective lifetime of an optic in large high-energy laser facilities. We present the damage performance on the rear surface of a large-aperture KDP crystal for 351 nm, 5 ns laser pulses. threshold than bulk damage after conditioning. Craters initiated on the scratch are found to increase with the shot number before filling the scratch. The experimental results reveal that damage initiation is mainly caused by extrinsic nanoabsorbers buried in the surface during the large-aperture laser operation.