Abstract: Poly(butylene adipate-co-terephthalate) (PBAT)-based biodegradable mulch films have garnered significant attention due to their environmental friendliness. However, their rapid aging under complex field conditions limits their practical service life. This study aims to elucidate the molecular mechanisms of photo-humidity synergistic aging in PBAT and to establish a solid theoretical foundation for selecting efficient light stabilizers suitable for different agricultural humidity environments.PBAT-based films were prepared via melt extrusion and film blowing, incorporating two types of light stabilizers: a triazine-based ultraviolet absorber (UV-1164) and a hindered amine light stabilizer (HS-944) at an addition level of 0.8wt%. Accelerated aging experiments were conducted using a xenon lamp weathering tester under two relative humidity conditions. The irradiance was maintained constant at 0.55 W/m²/nm at 340 nm. The aging behavior was systematically characterized through multiple techniques. Mechanical properties including tensile strength and elongation at break were measured using a universal testing machine. Chemical structural changes were monitored by Fourier transform infrared spectroscopy, with particular attention to carbonyl index evolution. Molecular weight and its distribution were analyzed by gel permeation chromatography. Thermal behavior was investigated by differential scanning calorimetry, and surface morphological changes were observed using scanning electron microscopy.High humidity conditions partially inhibited the photo-oxidation process, thereby delaying the deterioration of mechanical properties. The hindered amine light stabilizer HS-944 exhibited excellent and long-lasting stabilization performance under both humidity conditions. For PBAT films containing HS-944, the retention of elongation at break remained at 48% after 6 days of aging at 50% RH, and good toughness was maintained after 9 days of aging at 80% RH. Gel permeation chromatography analysis revealed that HS-944 effectively suppressed random chain scission and uncontrolled crosslinking, resulting in a consistently narrow molecular weight distribution throughout the aging period. The radical scavenging and regenerative cycling mechanism of HS-944 effectively interrupted the chain photo-oxidation reaction, providing sustained protection regardless of humidity level. In contrast, the triazine-based ultraviolet absorber UV-1164 exhibited pronounced humidity-dependent performance. Under low humidity conditions, its stabilizing efficiency was severely limited due to poor dispersion and molecular aggregation. PBAT films containing UV-1164 showed a rapid decline in elongation at break to 10% of the initial value after 6 days of aging at 50% RH, accompanied by a sharp increase in carbonyl index. Under high humidity conditions, enhanced molecular mobility and improved dispersion of UV-1164 within the PBAT matrix significantly increased its ultraviolet shielding effectiveness. The retention period of elongation at break was extended to 9 days, and Fourier transform infrared spectroscopy showed a much slower increase in carbonyl index. Differential scanning calorimetry analysis further confirmed that UV-1164 formed an effective protective layer under high humidity conditions. Scanning electron microscopy observations revealed that HS-944-stabilized films maintained relatively smooth surface morphology under both humidity conditions, while UV-1164-stabilized films exhibited pronounced surface cracking and aggregation under low humidity.Multi-scale analyses including mechanical testing, Fourier transform infrared spectroscopy, gel permeation chromatography, differential scanning calorimetry, and scanning electron microscopy collectively revealed the molecular structural evolution patterns of PBAT under photo-humidity synergistic aging. The findings elucidate the critical regulatory role of humidity in determining the effectiveness of different light stabilizer mechanisms. HS-944 provides humidity-independent long-term stabilization through efficient radical scavenging and regenerative cycling, while UV-1164 exhibits efficiency strongly dependent on humidity-controlled dispersion. These results offer theoretical guidance and technical support for optimizing the environmental adaptability and extending the service life of PBAT-based biodegradable mulch films under diverse climatic conditions.