Peroxyacetyl nitrate (PAN) is a critical secondary pollutant that drives photochemical smog and threatens ecological health. Despite persistent challenges in controlling photochemical pollution in China, the mechanisms governing seasonal PAN formation and its key driving factors under complex basin topography remain poorly constrained. In this work, we integrated a comprehensive one-year observational dataset, an observation-based box model, and interpretable machine learning to elucidate the seasonal formation pathways and meteorological drivers of PAN at an urban site in the Sichuan Basin. The annual mean PAN mixing ratio reached 0.60 ppbv, with a winter peak hourly value of 6.44 ppbv. Model simulations revealed distinct seasonal formation pathways: thermal decomposition–recombination cycling dominating summer formation (75.5 %), whereas precursor oxidation served as the primary source in winter (57.1 %). Sensitivity analyses indicated a persistent volatile organic compound-limited regime for both PAN and ozone formation year-round, while nitrogen oxides varied seasonally, suppressing PAN formation in winter but promoting it in summer. Feature attribution analysis further demonstrated an obvious seasonal shift from chemistry-dominated driving mechanisms during warm seasons to stronger meteorological influences in winter, where transport-related variables accounted for 61.0 % of the cumulative feature importance. During ozone pollution episodes, peroxyacetyl radical production increased by 77.0 %, accompanied by a 91.9 % enhancement in PAN decomposition, highlighting intensified PAN cycling driven by elevated photochemical capacity. These findings uncover seasonal regime shifts in PAN formation in complex basin environments, supporting seasonally tailored precursor reduction and coordinated PAN and ozone mitigation strategies.