The rapid transversal growth method based on continuous filtration is applied to effectively increase the Z-direction length of the potassium dihydrogen phosphate (KDP) crystal and improve the II-type plate yield of the unit crystal. Different doses of γ ray are used to irradiate on the II-type crystal. The optical performance and the laser induced damage threshold are tested. The research results show that different doses of γ ray irradiation do not change the lattice vibration modes within crystals, but can reduce the vibration intensity of the lattice at 912 cm-1. The defect types induced by the γ ray irradiation do not change, but the defect density increases with the increase of the irradiation dose. In addition, the laser induced damage threshold of crystal plates decreases with the increase of the irradiation dose.