Abstract: In order to understand the air quality in key polluted areas in China, fine particulate matter (PM_2.5) and ozone (O₃) were studied during the ‘Three-Year Action Plan to Win the Blue Sky Defense’ from 2018 to 2020 and the following year in 2021. More specifically, the characteristics of PM_2.5 and O₃ compound pollution were analyzed by using hourly monitoring data of PM_2.5 and O₃ in ‘2+26’ cities of Beijing-Tianjin-Hebei and its surrounding areas. In addition, the contributions of emission sources and meteorological factors to the long-term trend of PM_2.5 and O₃ concentrations were quantified based on KZ filtering method. The results showed that in recent years, the annual mean PM_2.5 and the 90^th quantile of maximum daily 8-hour average ozone (O₃-8 h-90^th) concentrations decreased in ‘2+26’ cities. The annual mean PM_2.5 concentration was 60, 57, 51 and 45 μg/m³, respectively. The higher concentration of annual mean PM_2.5 appeared in southern Hebei Province, Henan Province and southern Shandong Province. However, O₃-8 h-90^th concentration was 198, 195, 179 and 171 μg/m³, respectively. In 2018, O₃-8 h-90^th concentration was higher in Baoding, Shijiazhuang, Liaocheng and Jincheng (greater than 210 μg/m³), while Taiyuan O₃-8 h-90^th concentration was higher (192 μg/m³) in 2021. Futhermore, the long-term trend of PM_2.5 and O₃-8 h (maximum daily 8-hour average ozone) concentrations were more significantly influenced by meteorology. The long-term component of PM_2.5 affected by meteorology showed no obvious trend from 2018 to 2020 and a decreasing trend in 2021, while the long-term component of PM_2.5 affected by emissions showed a decreasing trend from 2018 to 2020 and no significant trend in 2021. The long-term component of O₃-8 h concentration affected by meteorology showed an upward trend from 2018 to 2020 and a downward trend in 2021, while the long-term component of O₃-8 h concentration affected by emissions showed a downward trend in 2018 and had no obvious trend from 2019 to 2021. Moreover, temperature and relative humidity have the greatest effect on the combination of PM_2.5 and O₃-8 h among the 11 meteorological factors. When the temperature and relative humidity increased together, it was more conducive to the simultaneous increase of PM_2.5 and O₃-8 h. The research shows that PM_2.5 and O₃ pollution in ‘2+26’ cities are significantly affected by meteorology, and the change of temperature and relative humidity has great significance for determining the simultaneous increase of PM_2.5 and O₃-8 h concentration.