The laser-induced damage threshold (LIDT) of a single-layer coating at the nanosecond (ns) regime is obviously lower than an uncoated substrate or a high reflectivity coating coated by the same material. To elucidate this phenomenon, we demonstrate the LIDT of three types of samples at 355 nm with 8 ns. High absorption defects are found at the film-substrate interface by comparing their LIDTs and damage morphologies. These defects originate from the substrate and appear during the coating process. Simulation results show that these defects, coupled to the coating, are mainly responsible for decreasing the damage threshold. Laser-induced damage to optical thin film coatings has been a limiting factor in the development of high-power laser systems. Pulsed-laser-induced damage to optical thin-film coatings at the nanosecond (ns) regime has been studied over the past five decades [1][2][3][4][5][6][7][8][9][10][11][12][13]. However, researchers still encounter several confusing problems. First, the laser-induced damage threshold (LIDT) of the antireflectivity (AR) coating [14,15], or the single-layer coating is obviously lower than the HR coating manufactured using the same materials or the uncoated substrate. Second, different rules about the LIDT of the single-layer coating were obtained [10,16,17] previously; some of these rules are contradictory. Third, the damage crater appears from the substrate if enough laser energy can reach the substrate in the AR coating or the single-layer coating. All these problems are still not solved. Thus, further studies are needed to elucidate the pertinent factors. To figure out answers for these problems, three types of samples are prepared for the LIDT experiments by a third-order harmonic generation of Nd:YAG laser (355 nm, 8 ns). These samples are the HR coating (reflectance >99%), the special coating (reflectance ∼57%), and the uncoated substrate (BK7). Here, the special coating is not an AR or a single-layer coating. This ensures that an ∼43% of the laser energy can reach the film-substrate interface. Moreover, the total thickness of the special coating is similar to the HR coating. The influence of stress on different coatings can be ignored. All substrates are polished and supersonic cleaned BK7 (Φ 50 mm × 5 mm) glasses. The surface roughness, the surface flatness, and the surface quality of the substrates are measured with a white-light Techo interferometer and a Leica optical microscope, respectively. The substrates have a low surface roughness (∼0.8 nm), fine surface flatness (∼0.1λ, 632 nm), and surface quality (20∕10 scratch∕ dig < 10), according to the MIL-O-13830A method [18].The coatings are prepared using a Leybold coater with an electron beam. The coating stacks of the two structures are named SubjHL 4 AL 16 AjAir and Subj1.06A1.06L 24 1.06AjAir (the special coating), respectively. "Sub" denotes substrate, "Air" denotes air, and "A," "H," and "L" denote Al 2 O 3 (1.69), HfO 2 (2.0), and SiO 2 (1.48), respectively. Each symbol represents an optical thickness of o...