Laser coating is a key component of high-power laser systems, such as inertial confinement fusion[1–3], and its laser-induced damage threshold (LIDT) directly affects the output power of high-power laser systems. Various defects in the coating and substrate can be sources of damage under laser irradiation. The effect of defects on the LIDT of coatings depends on the properties of the defects and coatings. For example, for coatings with partial or high transmittance at the laser wavelength (such as anti-reflection coatings, beam splitter coatings), the effect of substrate subsurface impurities on the LIDT is much greater than that of coatings with high reflectance at the laser wavelength[4,5]. Subsurface defects are generated during the chemical–mechanical polishing of substrates. Subsurface damage layers generally consist of a re-deposition layer (also known as the Beilby layer), a defect layer containing cracks and scratches and a deformed layer. Impurity particles are randomly distributed in the re-deposition layer, some of which are embedded in the scratches and cracks of the defect layer[6]. For a specified irradiated laser fluence, compared with a high-reflection coating, the laser intensity of a coating with partial or high transmittance is stronger at the substrate–coating interface, and subsurface impurity defects are more likely to cause damage under laser irradiation. In addition, the electron beam deposition process often requires the substrate to be heated, and subsurface impurities can easily migrate to the surface during heating[7,8]. Researchers have simulated the effect of impurity particles on the LIDT of substrates using Mie scattering theory and impurity defect absorption models[9,10]. Several studies have been conducted to investigate the formation process of the re-deposition layer[11,12], and reduce subsurface impurities by improving the polishing process[13], using ion beam etching[14–16], using acid etching[15–17] and so on. However, the mechanism by which subsurface impurities affect the LIDT of coatings with partial or high transmittance at laser wavelengths, such as plate laser beam splitter (PLBS) coatings, remains to be investigated.