<p>Aluminum-based functional films can effectively suppress near-infrared stray light in high-energy laser systems, maintaining ultra-high system cleanliness and extending the service life of optical crystals. Revealing the damage mechanism of films under stray light irradiation is one of the core methods to further improve their threshold to stray light. However, the process of stray light irradiation on films is extremely complex. In addition to involving the film surface, subsurface, and film-substrate interface, it also involves the nanosecond scale dynamic evolution of multiple physical fields, structures, morphology, and phase states of films. This paper establishes a multi-physical field simulation model for the near-infrared stray light irradiation process of functional films. Combined with in-situ experimental analysis, the effects of four key stray light parameters-energy density, spot diameter, incident angle, and pulse count on the irradiation damage behavior are studied quantitatively. Based on this, the near-infrared stray light irradiation damage mechanism of functional films is revealed, dominated by melting failure in the irradiation stage, thermal damage, and thermal stress-induced damage in the cooling stage. This research provides theoretical and experimental evidence for maintaining high cleanliness in high-energy laser systems, and offering engineering references for system light source design.</p>