In this study, silica sol with decane as a solvent was prepared by the sol modification and solvent replacement method. A film with good uniformity was coated on 50 mm×50 mm×10 mm square KDP crystals using the single spin coating method. The optical properties of the film coated on the KDP crystals were tested using a spectrophotometer. The peak transmittance value of the KDP crystal substrate coated with the third harmonic frequency and the fundamental frequency of the second harmonic frequency antireflective films could achieve more than 99.5% at 378 nm and 835 nm, respectively. The defect control in the preparation process of the coating film was realized by the combination of the filtration and the ultrasonic cleaning technology. The third harmonic frequency antireflective film with defect control was coated on a high-cleanliness fused silica substrate and pretreated by laser conditioning before testing. The laser-induced damage threshold of the third harmonic frequency antireflective film reached (14.0 ± 2.1) J·cm -2 measured by a laser with 355 nm wavelength and 3 ns pulse width.