Abstract: In recent years, PM_2.5-O₃ pollution has become a major feature of air pollution in China. This study took Hengshui city as an example to investigate the prevention and control characteristics of composite pollution in the Beijing-Tianjin-Hebei region. Based on the observation data of Hengshui Atmospheric Environment Monitoring Super Station from May 2021 to September 2022, the study days were classified according to pollutant concentrations, and the correlation between PM_2.5 and O₃ under different pollution types and photochemical levels was explored. The results showed that: (1) Clean days (low PM_2.5-low O₃) accounted for the largest proportion in Hengshui from May 2021 to September 2022 (48.3%). (2)The daily average PM_2.5/CO ratio in Hengshui showed a seasonal trend of winter (0.09) > spring (0.08) > autumn (0.06) > summer (0.04), with stronger oxidation in spring. (3) PM_2.5 concentrations, PM_2.5/CO, and the proportion of secondary aerosols in PM_2.5 increased with increasing levels of photochemical activity. (4) Synergistic growth periods mainly occurred when wind speed was 1.3-1.8 m/s, temperature was 25.8-32.1 ℃, and relative humidity was 34.3%-60.5%, with the largest proportion of hours with synergistic growth events occurring at mild photochemical activity levels (52.9%). This indicated that low wind speed, high temperature, and low humidity were favorable for the occurrence of PM_2.5-O₃ composite pollution. The formation of secondary aerosols in PM_2.5 could be enhanced at light photochemical activity levels. The study indicated that O₃ promoted the formation of secondary components in PM_2.5, and meteorological conditions were also one of the important factors contributing to PM_2.5-O₃ composite pollution.