Abstract: Potassium dihydrogen phosphate (KDP) crystals were baked at different temperatures in the same procedure. Samples from similar locations of as-grown 500-mm-class KDP crystals were used to measure the transmission spectra and laser damage thresholds. The results prove that the transmission spectra are not affected by thermal conditioning, and the laser damage thresholds of samples after annealing at 140 ℃and 160 ℃ are not significantly different from that of the unconditioned sample. However, for samples annealing at 150 ℃ a 1.4 times increase in the laser damage threshold is observed at 1, indicating that the annealing temperature of150 ℃ is the best for elevating the laser damage threshold of KDP crystals.