Peroxyacetyl nitrate (PAN) can effectively indicate photochemical pollution, and also plays a vital role in regional oxidant balance. One-year continuous monitoring of PAN in a coastal city of southeastern China was investigated. The mean concentration of PAN in winter (0.64 ppb) was close to that in autumn (0.73 ppb), indicating that photochemical pollution was still non-negligible in the cold season. The peak occurrence time between O₃ and PAN had a delay of 1–2 h in four seasons, due to the rapid decomposition rate of PAN in midday. Emission sources of the precursors are located to the south of the monitoring site, so high concentrations of PAN and O₃ are frequently observed under southerly wind conditions. The air mass with low concentration of PAN (0.22–0.34 ppb) and O₃ (18.17–23.67 ppb) originated from the ocean with less anthropogenic air pollutants. Continental air mass with high PAN concentration might be related to the contribution of heterogeneous reactions of PM_2.5 to the promotion of PAN formation. In the polluted case, PAN concentration was often higher than 1.0 ppb and reached the peak of 4.2 ppb, suggesting the influence of photochemical reactions and local accumulation. High concentrations of HONO and sufficient ultraviolet radiation might be the main factors for rapid photochemical production of PAN. Besides, the lifetime of PAN in winter under the condition of high PM_2.5 concentration (≥35 μg^.m^-3) was up to 3.246 days. This study provided insights into photochemical mechanism and pollution process in a coastal city of southeastern China.